Add a tags plugin to manage the tags used in the site.

Currently has the following features:

 - List all the tags (alphabetically or based on occurence frequency)

 - Replace multiple tags with a single tag, given a list of posts

 - Run the merge command in a test mode, to see the output before
   modifying a post.

This plugin is inspired by [jtags](https://github.com/ttscoff/jtag)

The list option can be used for writing [editor plugins](https://gist.github.com/punchagan/6629020), to help add tags when writing new posts. The merge option can be used to clean up. 

*NOTE*: This is a work in progress, and suggestions are welcome.