Mkdocs-macros-plugin

* Added: Info on pluglets, on GitHub index page.
* Added: Contributing and Help pages, in documentation

Warning: Breaking Change

* Fixed: impossibility to use imported Jinja2 macros, without `with context`
clause (81). Now macros are imported as global.
* **Removed: Do not define macros as variables any longer, but as macros.**
- incorrect: `env.variables['foo'] = foo` (though it should still work)
- correct: prefix declaration with `env.macros`
or `env.macros['foo'] = foo`
* Added: Changelog is also part of documentation
* Added: Documentation moved under Material them, slate variant (dark)
* Added: Amended documentation (test install, discussions)

* Added: Changelog (Fixed 82)

* Fixed: display better message in case of macro syntax error
(line_no, message, incriminated line in file).
Traceback was useless with that specific exception, and has been removed.

* Added: Possibility (for large projects) to exclude a markdown page
from macro rendering, with `ignore_macros: true` in YAML header
(fixed issue 78, and answered discussion 79)

* Added: Files object to the mkdocs-macros environment (fixed 80)
* Fixed: Documentation errors or omissions
* Fixed: Do not install pluglet mkdocs-macros-test by default (50)
In order to do testing,
type: `pip install 'mkdocs-macros-plugin[test]'`