This introduces a new rbtools.config module that's responsible for all

configuration management for RBTools. It handles scanning for

.reviewboardrc files, loading them, and providing an object that can

be used for looking up settings.
Half of this is code moved out from rbtools.utils.filesystem and

updated (for typing, cleanup, and moving out config merging support to

the other half).
The other half is the new wrapper for configuration state.
rbtools.config.config.RBToolsConfig (which is not at all a

redundant-sunding namespace) is an object that wraps a loaded

dictionary. This provides a few advantages over the old dictionary:

For any RBTools-provided settings, we now have type annotations.
We also have default values, which no longer have to live in Option

  or be handled by codepath-specific .get(..., default) calls.
Keys can be accessed via dictionary key access as before, or via

  attribute access now.
All keys are now documented in one place, with version information.

Custom settings not known to the RBTools codebase are still loaded and

can be accessed.
This will make it easier to create and maintain settings going forward.
Some initial plans for this change were scrapped, such as type

validation on load and config-driven option construction. For the latter

case, the thought was to make parameters, help output, and defaults all

come from the central config, but upon reflection, it was ultimately

decided that that would box us in if we ever wanted to make larger

changes to options. These will remain independent, but a future change

will update options to get their defaults from the central

configuration.
As of now, code is still using the now-deprecated functions for loading

configuration. That will be updated in a separate change.

Unit tests pass.
Used this to post this change, verifying it relied on my aliases and

my Markdown setting in my home directory and the repository settings

in the repository.