log: Add support for Logging's 'extra' keyword argument

Python 2.5 added the optional keyword argument *extra* for providing an optional dictionary of user defined attributes (See: [Python 2.7 source](http://hg.python.org/cpython/file/2.7/Lib/logging/__init__.py#l1252)).  

This change mirrors Python's _log so that plugins with custom loggers can utilize this keyword without breaking Limnoria's internal logging.