Automatically generate whenever file contents change

[aprescott]

There needs to be an optional automatic publishing mechanism that detects new or changed posts.

The desired outcome is that a new post can be copied over to _posts manually (say, using rsync), and the site will be generated automatically.

If draft preprocessing is implemented for #9, then the additional behaviour is that editing/adding a file in _drafts should cause the site to be regenerated.

Candidates here are something more at the system level, like an inotifywait wrapper (which is basically how Second Crack does it), or something closer to the application with, say, directory_watcher.

[aprescott]

Up for debate is whether to handle changes outside of _posts and _drafts as part of this. Probably, yes.