Replace print/warn with __print__/__warn__

The benefit of having a replacement method for these two items is that
we can then easily silence all output by simply changing the
$RCFiles::__QUIET__ variable to a true value. Alternately, all output of
the program can easily be internally copied by attaching a wrapper
function to __print__ or __warn__.
Make subs more consistent/shorter in `rcfiles`

New subs:
  * __good - Print a positive/informational message. Return __success__.
  * __bad - Print a negative/nonfatal error. Return nothing.
  * __die - Print a fatal error. Return __failure__.

Renamed subs:
  * __print__   -> __print
  * __warn__    -> __warn
  * __chdir__   -> __chdir
  * __findsub__ -> __findsub

The general rationale is that private subs should be prefixed with `__`,
but the suffix of `__` is unnecessary, and could instead be used to
indicate a constant. `__main__` is an exception to this as it is a
special method that should only ever be called once.

Additionally, the way error messages and informational messages are
printed could change at any time, and it would be much better to have it
be done through a consistent method so that, should I choose to do so,
it will only need to happen in once place. Also, being able to print a
fatal error and return on the same line is a nice new benefit.
Add `reset` subcommand to `rcfiles`

`reset` will clear all of the modifications to the local copy of the
rcfiles repository. This includes clearing out the staging index from
git, removing untracked (but not ignored) files, and resetting all
changes to the latest HEAD version. There is also an informative comment
on the `rcfiles status` output to showcase this option.