* 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

* cargo fmt & clippy

* Skip anchor checking for URL with prefix in config

Links that start with a scheme (e.g., `tel:18008675309`) inadvertently
had a URL prefix prepended. Previously, only `mailto:` was handled, but
given the sheer number of [registered URI schemes][uri-schemes], a loose
pattern matcher is used to detect schemes instead.

External links, as identified by the renderer, are now limited to `http`
and `https` schemes.

Fixes #747 and fixes #816.

[uri-schemes]: https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml

* Bump pulldown_cmark to version 0.6.0

* Rename headers to headings

Close #693

Close #686

* 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

Justification for this feature is added in the docs.

Precedent for the precise syntax: Hugo.

Hugo puts this syntax behind a preference named headerIds, and automatic
header ID generation behind a preference named autoHeaderIds, with both
enabled by default. I have not implemented a switch to disable this.

My suggestion for a workaround for the improbable case of desiring a
literal “{#…}” at the end of a header is to replace `}` with `}`.

The algorithm I have used is not identical to [that
which Hugo uses][0], because Hugo’s looks to work at the source level,
whereas here we work at the pulldown-cmark event level, which is
generally more sane, but potentially limiting for extremely esoteric
IDs.

Practical differences in implementation from Hugo (based purely on
reading [blackfriday’s implementation][0], not actually trying it):

- I believe Hugo would treat `# Foo {#*bar*}` as a heading with text
  “Foo” and ID `*bar*`, since it is working at the source level; whereas
  this code turns it into a heading with HTML `Foo {#<em>bar</em>}`, as
  it works at the pulldown-cmark event level and doesn’t go out of its
  way to make that work (I’m not familiar with pulldown-cmark, but I get
  the impression that you could make it work Hugo’s way on this point).
  The difference should be negligible: only *very* esoteric hashes would
  include magic Markdown characters.

- Hugo will automatically generate an ID for `{#}`, whereas what I’ve
  coded here will yield a blank ID instead (which feels more correct to
  me—`None` versus `Some("")`, and all that).

In practice the results should be identical.

Fixes #433.

[0]: a477dd1646/block.go (L218-L234)

Closes #607

This fixes #569 .

`markdown_to_html` is heavily refactored, header-related things is
handled in a second pass.

* add id to continue reading p tag

this commit contains two refactors:
- extract custom link transformations into a function.
- separate some trivial markup generation.

To override all child pages

Closes #397
Tests from PR #434

Fix #430

Closes #403

As mentioned in #376

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.

Close #115