Organize plugins into categories.

It's difficult to skim through the plugins on the plugins page. This PR organizes the plugins into generators, converters, tags, filters, collections (groups of plugins in one git repo), and other (Jekyll extensions like the asset pipelines). A little improvement while https://github.com/mojombo/jekyll/issues/1272 gets worked out.

Also made some copy edits:

- added descriptions where there weren't any
- tried to remove some of the superfluous text like "a Jekyll plugin that..." without changing the text too much
- corrected capitalization and normalized punctuation
- removed lengthy instruction from the plugin description
- rewrote some non-descriptive descriptions (e.g., "time-ago: a time-ago liquid filter")

While going through plugins I found these three that I think could go or stay:

- [Flickr](https://github.com/reagent/fleakr): Embed photos from flickr right into your posts.  
  *This is a Ruby API -- not sure it has anything specifically to do with Jekyll.*
- [Upcase Converter by Blake Smith](https://gist.github.com/449463): An example Jekyll converter.  
  *A very simple example. Could still be useful, but doesn't add anything to the converter description above.*
- [Draft/Publish Plugin by Michael Ivey](https://gist.github.com/49630)  
  *No documentation, usage instructions*

I removed Fleaker but left the other two in the PR.