Tags

## Tags
Adds support for displaying and adding tags to posts. 

![tags](https://cloud.githubusercontent.com/assets/8317250/4896626/04dd0586-63fd-11e4-9fdc-9470c17b320e.gif)

### Adding tags
* Edit `blog/_data.json` and add the tag to a `tags` array inside the post object:
```json
"tags": ["tag1", "tag2"]
```
* Add a new empty **_name_of_tag_**.jade file inside `tags/`.

![new_tag](https://cloud.githubusercontent.com/assets/8317250/4896630/1ca7610c-63fd-11e4-9b96-76619e926eaa.gif)

### Notes
* There's some code repetition because I added the tags to both the main __layout__ and __author__ pages.
* The above can be easily fixed, however, adding something like `shared/mixins.jade` and creating some utilities. 
* I encourage you to consider using mixins to tidy this up as well as other things like, 
  * the meta tag inferno in `_layout.jade`
  * the html5shiv
  * the post loop so as to share some of the functionality with `_author.jade`, etc.
* If you agree, I am willing to commit the above in another pull request.