improvement: expose the provider of gitlab, bitbucket, and github (useful on cli)

this way you can do:

    git issues cli
    repo.provider

and get low-level access to the object

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
Merge pull request #16 from gittalk/expose-provider

improvement: expose the provider of gitlab, bitbucket, and github (usefu...