* maybe_slugify() only does simple sanitation if config.slugify is false

* slugify is disabled by default, turn on for backwards-compatibility

* First docs changes for optional slugification

* Remove # from slugs but not &

* Add/fix tests for utf8 slugs

* Fix test sites for i18n slugs

* fix templates tests for i18n slugs

* Rename slugify setting to slugify_paths

* Default slugify_paths

* Update documentation for slugify_paths

* quasi_slugify removes ?, /, # and newlines

* Remove forbidden NTFS chars in quasi_slugify()

* Slugification forbidden chars can be configured

* Remove trailing dot/space in quasi_slugify

* Fix NTFS path sanitation

* Revert configurable slugification charset

* Remove \r for windows newlines and \t tabulations in quasi_slugify()

* Update docs for output paths

* Replace slugify with slugify_paths

* Fix test

* Default to not slugifying

* Move slugs utils to utils crate

* Use slugify_paths for anchors as well

* Let toc is visable through Page & Section variables in templates

* Removed the current toc variable from page & section

* Bump pulldown_cmark to version 0.6.0

* Rename headers to headings

* fix the issue of generating the search index for multiple language

* updat docs for generating the search index for multiple language

* fix failed tests

* add tests for the search index of multiple language

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 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

Closes #674

To ensure we will get the right `content`
directory.
Fix #629

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 #537

Fix #396

Closes #485

Close #446
Close #260
Close #478
Close #284
Close #480

Fix #205

Push summary handling into Markdown parsing, identifying the presence
of one by giving its length in the rendered markup.

Hopefully a better fix for #376.

Also fix tests

Close #367, #372

This commit removes the option to sort by order and also removes
`page.next` and `page.previous` variables.  Instead, pages can be sorted
by two methods `date` and `weight`.  The Tera `reverse` filter will
reverse either of those sorts, so the old `order` behavior can be
achieved by using the `reverse` filter with `weight`.

In place of the `previous`/`next` variables, this commit adds the
`page.earlier`/`page.later` variables (which are set when the page is
sorted by date) and the `page.heavier`/`page.lighter` variables (which
are set when the page is sorted by weight).  These variables have the
advantage of not having confusing semantics when the `reverse` filter is
used.