When we enable a plugin (when it's provided in the --enable-extensions)
plugin, we need to remove it both from the extended default ignore list
and the plain ignore list.
Closes#239
This relies on two things:
1. Properly configuring flake8-import-order to use that style
2. Properly configuring flake8-import-order to know that flake8 is our
application name.
We move the logic to add or remove a plugin from the default ignore
list to individual methods on the Plugin class (Plugin#enable,
Plugin#disable) and use that when registering and parsing options.
If the plugin is off-by-default, Plugin#register_options will use
Plugin#disable. When parsing options via Plugin#provide_options, if
the plugin has been specified in --enable-extensions then it will be
re-enabled via Plugin#enable.
Let's catch exceptions, log them, and re-raise as a FailedToLoad
exception. This also refactors the actually loading of plugins into a
private method on the Plugin class.
