new plugin type: postContentPlugins

I don't intend for this to be merged as-is, but rather to start a discussion. I started running into some problems that would be solved by something such as #144, and did some hacky workarounds that I wasn't proud of.  Then I wanted to do something that required access to the contentTree, but wasn't really a generator.  So I thought on it a bit, and came up with the "postContentPlugin". 

After the ContentPlugin has run and after the tree has been attached to the content instance, a postContentPlugin that matches a pattern against the instance.filename will run to do further processing on the page instance.  Examples of use:

1. modifying the html to fix a bug in the content plugin: https://github.com/mattly/lyonheart.us/blob/master/plugins/fix_old_typogr_bug.coffee
2. pulling in extra metadata from somewhere else: https://github.com/mattly/lyonheart.us/blob/master/plugins/metadata.coffee
3. modifying both the page html and the metadata, taking markdown footnotes from the html and moving them to metadata: https://github.com/mattly/lyonheart.us/blob/master/plugins/footnote-extraction.coffee

This solution is extra-nice for the third one, previously I was "extending" wintersmith-showdown [by chaining its render](https://github.com/mattly/lyonheart.us/commit/8d3d60d8fe4dd5b6c9cc2053e035de6a4a395a62#diff-29347f2c6fe55f298393a02f0f052c1bL31) but that's really hacky and doesn't scale to many plugins well.  I think this solution is more elegant.

Anyway, let me know what you think. I like working with wintersmith so far and I have a lot of ideas for things that extending it this way will let me do.