Add collaborators as reviewers option

Pair programmed with @Vagabond.

Fixes #16

When the 'collaborators_as_reviewers' = true option is added
to the config, the list of collaborators on the project will
be used instead of the hardcoded reviewer list. This allows
you to not have to keep a hardcoded list of reviewers up-to-date.
Allow configurable approval/disapproval tokens

Pair programmed with @Vagabond.

Instead of hardcoding r+, r=me, and r- for approval/disapproval
tokens, we (try to) read the list of allowed tokens from the config.
If the tokens are not overridden, we simply default to the current
implementation.