Add dependencies to markdown libraries
More flexibility in Gruntfile

Allows plugins to define additional dependencies, that need to be
loaded before they can be loaded themselves. These will be bundled with
the destination min.js file.

arethusa.comments is doing this with its markdown libraries.
Add a markdown module to arethusa.comments
Parse comments with markdown
Add placeholder hint for markdown input
Merge pull request #418 from latin-language-toolkit/markdown_comments

Markdown comments