Remove unnecessary dict.get() calls when retrieving config values

Now simply use dict indexing instead. Uses simpler and more conventional
Python syntax for directly accessing a value in a dict. A small micro
optimization for keys known to exist as they're specified in
isort.settings.default. Will also more easily expose typos and
programming errors as an incorrect key now results in a KeyError instead
of silently falling back to a default value.

There remain a few config.get() calls as changing them causes test
failures. I wasn't sure which case they fall under:

* Truly optional values that may not exists, dict.get() is the best choice here
* Should be specified in settings.default
* Exposing an unnoticed error