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

This reverts commit 1815155c1d.

Closes #530

Fix #616. Add brief documentation for how to use it.

This reverts commit 5fd7bf7e61.

Apparently it is not just no longer necessary to use classic
confinement, but actually impossible, i.e. snap emits an error that the
zola snap is not compatible with `--classic`.

690b3f9 updated the required version in Cargo.lock and in the CI config files, but not in the installation instructions.

If you don't install with classic confinement, you'll get permission errors trying to create sites with `zola init`.

Changed arch linux from h2 to h3

I updated the install instructions (snap, scoop, chocolatey) and added the ones where Gutenberg already got renamed to Zola also to the top of the document.

Should yaourt be replaced by one of the other aur-helpers (pikaur, yay,...) as the development for it is stalled/discontinued for quite some time now although it is still working?

Scoop: https://github.com/lukesampson/scoop/blob/master/bucket/zola.json
Chocolatey: https://chocolatey.org/packages/zola
Snap: https://snapcraft.io/zola

Switch to an `Option<usize>` for the serialized value of `rss_items`.
This lets us just set a blank value in the configuration and thereby
include *all* items.

This is a backwards-compatible change; it does not affect the behavior
of existing configurations.

Fixes #468. Closes #471.

- unless forced, only use colors in tty
- support CLICOLOR and NO_COLOR vars
- switch to termcolor crate to handle output coloring

Close #115

- Fix some typos
- Some small changes to documentation to make it more clear
- Change the themes list from text to links

Closes #254

* Add ignored_content to the Config structure.
* Use the GlobSet crate to parse the glob patterns into a matcher, which
  is created once at program initialization. If there are no patterns in
  ignored_content, an empty globber is created, which excludes no files.
  This is consistent with the existing behaviour of Gutenberg, before
  this feature was added.
* Bail if there are any errors in the glob patterns.
* Add a call to the globber in page.rs to actually do the filtering.
* Update documentation.

A note on the Config structure
------------------------------
* I had to remove the PartialEq derive from the Config structure as it
  does not work for the GlobSet type. No harm is done, Config does not
  need to be PartialEq anyway, since there is no need to sort Configs.
* The implementation follows the pattern of the existing config settings
  in that it uses an Option<...>. This would appear unnecessary, in that
  an empty vec could be used as the default, but it appears to be needed
  by the TOML parsing. A better approach would be to use a separate
  SerializableConfig and map to/from a Config struct. This would also
  allow the elimination of most, if not all, of the other Options in
  the Config structure, but that ought to be another PR.

Separate interface and base_url for serve

You can now have a different base_url and listening interface.
The `gutenberg serve` cmd now accepts `-u`/`--base-url`, but it defaults to `127.0.0.1`.