Post viewing enhancements!

This branch/pull contains two major features:

1. Implementation of the "single post template" plugin within Largo to allow for custom templates for posts (in addition to WP's built-in Page support), along with one new Post Template for "full width" posts.

2. Implementation of "Clean read" mode on post pages. There's a new theme option to display a "clean read" link either in the byline or in the page footer. Upon click, "clean read" mode is enabled and a new close button is fixed to the upper-right of the screen (hidden by admin bar if you're logged in) to get out of it. Hitting escape also exits from the mode.

Not sure you'll love my theme option implementation, but I decided I didn't like how neimanlab had the link in the sidebar (though you could put one there — any <a> with the class of "clean-read" will work, so you could easily do a text widget with the link).