* Add support for underscore in names to prevent interference w/ gemoji
* Disallow underscore at end of name to prevent interference w/ emphasis
Related to GH-651.
Previously, brackets were only allowed in labels when escaped,
making it impossible to use media with resources (links, images),
or nested directives.
This adds support for up to three levels of nesting in a label.
This also removes a hidden flag that was not used anymore.
Closes GH-7.