Added the abiltiy to add your own custom formatter for pygments

This is really two new features.
1. The ability to inject arbitrary classes or modules into your source at runtime.
2. The ability to inject your own pygments formatter. 

The main entry point is here:
https://github.com/jbcurtin/pelican/blob/master/pelican/settings/base.py#L122

This configures a packaga that can be imported like so:
>>> from pelican.settings.conf import settings as pelican_settings
>>> pelican_settings.PATH
"/Users/user/path/to/execution"
>>> pelican_settings.PYGMENTS_FORMATTER
"jbcurtin.formatters.HTML5Formatter"