this is the first step in building a new `zola index` command that allows building search indexes as a stand-alone task.

the new Subcommand in src/cli.rs outlines the planned api.

the biggest thing it will be providing compared to current code is optional indexing with tantivy.

this commit doesn't add tantivy or any new functionality other than allowing the existing search indexing to be performed as a subcommand. (passing --index-type tantivy will trigger unimplemented!() panic)

hyper is already included in Zola due to the reqwest dependency (used
in the link_checker and templates components). Replacing Actix with
hyper in the serve command reduces the number of dependencies and
slightly improves build times and binary size.

* fix(init):  handle already existing path

* chore: add tests

Presently when you `^C` in `zola serve` it is painted with the same
color as the previous message. This PR always ensures to reset the color
in colorize, before writing the newline.

* ADD: reload when themes change

close #713

* MOD: test can_detect_kind_of_changes

Closes #773

* zola init works without argument (defaults to current directory)

* zola init works with existing folders (no non-hidden files permitted)

* Update zola init documentation

* [docs] Optional argument is on a separate line for clarity

* Add tests for init subcommand's respect of existing folders

Closes #766

* Allow ignored_content to support markdown files

* Add test for markdown supported ignored_content

Closes #752

Closes #726

Clippy is returning some warnings.  Let's fix or explicitly ignore
them.  In particular:

- In `components/imageproc/src/lib.rs`, we implement `Hash` explicitly
  but derive `PartialEq`.  We need to maintain the property that two
  keys being equal implies the hashes of those two keys are equal.
  Our `Hash` implementations preserve this, so we'll explicitly ignore
  the warnings.

- In `components/site/src/lib.rs`, we were calling `.into()` on some
  values that are already of the correct type.

- In `components/site/src/lib.rs`, we were using `.map(|x| *x)` in
  iterator chains to remove a level of indirection; we can instead say
  `.copied()` (introduced in Rust v1.36) or `.cloned()`.  Using
  `.copied` here is better from a type-checking point of view, but
  we'll use `.cloned` for now as Rust v1.36 was only recently
  released.

- In `components/templates/src/filters.rs` and
  `components/utils/src/site.rs`, we were taking `HashMap`s as
  function arguments but not generically accepting alternate `Hasher`
  implementations.

- In `src/cmd/check.rs`, we use `env::current_dir()` as a default
  value, but our use of `unwrap_or` meant that we would always
  retrieve the current directory even when not needed.

- In `components/errors/src/lib.rs`, we can use `if let` rather than
  `match`.

- In `components/library/src/content/page.rs`, we can collapse a
  nested conditional into `else if let ...`.

- In `components/library/src/sorting.rs`, a function takes `&&Page`
  arguments.  Clippy warns about this for efficiency reasons, but
  we're doing it here to match a particular sorting API, so we'll
  explicitly ignore the warning.

* Add hard_link_static config option.

* Copy or hardlink file depending on an argument.

Modify the call sites for `copy_file` to account for the extra argument.

* Plug the config setting through to copy_file.

Don't apply the config option to theme's static directory.

* Update documentation.

* Backticks make no sense in this comment.

* Addressing PR comments.

* Be consistent with argument naming.

* Add --open flag to serve command, to open URL in default browser

* Mention serve --open flag in CLI documentation

Closes #712

* actix_web::fs is now found in the actix-files crate.

* NotFoundHandler is now implemented using ErrorHandlers and the
path to 404.html is provided to the not_found() function using
actix_web::App::data. This setup allows for additional StatusCodes
to be handled in the future, if desired.

* Cleanly serving index.html files for requests to directories now
appears to be possible simply by using index_file("index.html") in
the static files service, so handle_directory() is removed.

* Add check subcommand

* Add some brief documentation for the check subcommand

* Start working on parallel link checks

* Check all external links in Site

* Return *all* dead links in site

To also process the ones found in templates

Closes #669

This reverts commit 1815155c1d.

Closes #667

Closes #607

trim_{start, end} is introduced in rust 1.30.0 and
trim_{left, right} is deprecated since 1.33.0.

Closes #576

Closes #554

Closes #385