Parse group description

[xi]

Fixes #520

@pascalduez had proposed to use an object for group instead of adding the groupDescriptions key. However, I found that to many places rely on the fact that group is a string, so I think this is less invasive.

[coveralls]

Coverage increased (+0.05%) to 94.727% when pulling 2b3ec76 on xi:feature/group-description into a544986 on SassDoc:master.

[pascalduez]

This will need more testing.

You went with the white-space version finally.

What happens when used as follow:

/// @group name description

////
/// This is a poster comment.
/// @group name
/// A group description
/// @author Hugo Giraudel
////

[xi]

I expanded the test. I realized that I had to add a file under annotations/ to make it work. Not sure if this is really the best approach.

/// @group name description

It results in a group with name "name description". I did not want to change this for backwards compatibility.

[pascalduez]

Could you please just add a comment in here explaining why it was added, and that it's not a real annotation etc. basically we shouldn't add it, but I don't want to delay your feature further, our tests will need a good refactor anyway.

[pascalduez]

Thanks!