Update extensions for Extension.__init__ refactor

Fixes #325. All extensions can now accept a dict of configs or
**kwargs, not just a list of tuples. Third party extensions may want
to follow suite. Extensions may only accept keyword arguments
in the future. These changes still need to be documented.

A couple things of note:

The CodeHilite extension previously issued a DeprecationWarning
if the old config key `force_linenos` was used. With thins change,
a KeyError will now be raised.

The `markdown.util.parseBoolValue` function gained a new argument:
`preserve_none` (defaults to False), which when set to True, will
pass None through unaltered (will not convert it to False).