Added tags as data attribute on post and named classes on tag labels

[ben5516]

I think it's beneficial to be able to target specifically tagged posts. For instance, if I wanted to tag certain posts as sponsored and then give them a blue background, I could do the following in css:

section[data-tags~=promoted] {
  background: #c3e9ff;
}

Likewise, I can target the tag label as well:

span.tag-promoted {
  background: #c3e9ff;
}

[coveralls]

Coverage decreased (-8.78%) to 87.88% when pulling fa05819 on ben5516:post_tags into 0ef9ed3 on jipiboily:master.

[msevestre]

@jipiboily could you trigger Travis again for this PR?

[jipiboily]

@msevestre just did. Do you know a way for me to give you access to that on Travis by any chance?