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)
Flakes cleanup.
Add tests for list and merge features in tags command.
Use comma separated list of tags for tags --merge
Add --add and --remove options to the tags command.
Use post.tags instead of post.alltags to prevent reordering tags.
Add --sort option to the tags command.
Improve usage help.
Add --search option for tags command.

Currently does a basic term in tag and re.match.
Change the files only when tags have changed.
Commit missing tags test file.
Minor fixes to get tests running on Py2.6 and Py3
Move the private functions to the end of the file.
Move all checks for is_two_file to _replace_tags_line function.
Add a dumb class that does tf-idf based tagging for a site.

This is too dumb to be used, mostly just an academic exercise.
Improve the tf-idf auto tag class to use nltk, if available.

- tokenize using nltk,
- remove stopwords
- stem the words, and use those to calculate tf idf
- tag the post!
Modify IDF calculation to give higher scores to existing tags.
Minor cleanups and version bump.
Merge pull request #1 from punchagan/tags

Add an experimental tags plugin