Closes #937

* Change continue-reading to use a span element instead of named anchor

* Fix all tests

For the site integration tests, we have a file of common code which is
used by multiple files in `tests/`.  However, not all functions in
this file are used by all files in `tests/`.

As Cargo compiles each `tests/*.rs` file as a separate crate, this
means that some of these crates end up with unused code.  Rust notices
this and prints a warning.

Let's tell Rust that we don't care about dead code in this file so
that the warning is not printed.

The issue with the check_site test hanging and timing out seems to
be related to a similar reqwest issue, which was ultimately due to
an upstream bug in tokio and may be fixed in tokio 0.2.7 onward.

* Restore #![feature(test)] and extern crate test; statements, which
were mistakenly removed as part of the Rust 2018 edition migration.

* Fix rendering benchmark's usage of RenderContext. 6 parameters were
provided when 5 were expected.

Closes #366

"[…] `&` normally indicates the start of a character entity reference or
numeric character reference; writing it as `&` […] allows `&` to be
included in the content of an element or in the value of an attribute."

From: https://en.wikipedia.org/wiki/HTML#Character_and_entity_references

* cargo fmt & clippy

* Skip anchor checking for URL with prefix in config

* 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

Closes #766

* Allow ignored_content to support markdown files

* Add test for markdown supported ignored_content

Closes #756

Closes #726

Part fix of #742

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.

* chore: Update glob to 0.3

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

* chore: Update ws to 0.8

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>

* 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.

This feature was originally reported in #257 and got fixed in
3a2dab59743e8d193a80ddb1aa1932e1b5462322, however it seems this
got lost during development.

Closes #661
Ref #257

Close #633

Split sitemap when it is getting too big