Optionally delete the pull-request source branch

If `delete_source_branch` is set to true in the config, bors will
automatically try and delete the source branch once the pull-request has
been succesfully merged. When no value is set in the config, bors will
_not_ delete the feature branch.
Merge pull request #28 from basho/feature/delete-feature-branch-on-merge

Optionally delete the pull-request source branch