Make configurable from Pelican configuration file

Allow user to configure the class addtions using the BOOTSTRAPIFY keyword in the Pelican configuration file. The user can pass a dictionary to BOOTSTRAPIFY with {'css-selector': ['list-of-classes']} and the plugin will now append 'list-of-classes' to whatever tag matches 'css-selector'. This is achieved by using select() instead of findAll() in replace(). For backward compatibility, if BOOTSTRAPIFY is not defined the plugin will act as it does now (for table and img).

Also, add a unit test, update README, and do some PEP8 formatting to the source code.

(Sorry if I did something wrong, this is my first pull request ever!)

Chamon