Mkdocs-macros-plugin

* 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]'`

* Starting point