Add a debug CLI command

[ogizanagi]

Introduce a new debug:stenope:content that'll be useful in multiple ways, to debug, list, filter and inspect contents managed by Stenope:

List

bin/console debug:stenope:content "App\Model\Page"

Show

bin/console debug:stenope:content "App\Model\Index" README

(change verbosity to display more)

Order & Filter

bin/console debug:stenope:content "App\Model\Article" \
   --filter="not:outdated" \
   --filter="slug contains:rebranding" \
   --order="desc:date"

---

There are some minor improvments / changes included in this PR as well:

• handle fragment (anchors) when linking content between them (see how we reference another document with an anchor in the documentation changes in this PR)
• added the ability to provide a filter function for a specific property (it allows the --filter=field contains:text feature exposed in the command)
• added the TableOfContent class. Small BC break (change your typehints from Headline[] to TableOfContent), but it allows to dump a simpler representation of the TOC when inspecting a content in non-verbose mode. It may also be useful later in case we need to introduce more data/config inside the TOC.
• ContentManager::getContents now returns the slugs as keys

TODO

• Add some functional tests for the debug command
• Add unit & functional tests for resolving links with anchors
• Add unit tests for filtering with a callable for a specific property
• Add tests to reflect the keys for ContentManager::getContents
• Later: interactively ask the class if not provided
• Later: allow partial classname if no conflict and ask between matches otherwise
• Later: dedicated & more powerful parser classes for orders & filters

[ogizanagi]

Merged in 0a01b1b