Add ability to load refresh_token from praw.ini

[JKauff]

I have a bot that does not run 24/7, only uses 1 reddit account, and has no additional configuration. With this change, I can store refresh_token along with client_id and client_secret in praw.ini and simply refresh_access_information() on bot startup to retrieve a new access_key.

[bboe]

Looks great. Could you please also update the config file doc to include a description of the parameter?
https://github.com/praw-dev/praw/blob/master/docs/pages/configuration_files.rst

Thanks for the contribution!

[JKauff]

Lol oops. Sorry for all of the reverts/commits. I'm new to git. I think it's ready now.

[bboe]

Merged as 7ffcb63. Thanks for the contribution.

Tip for your next pull request:

• it's a bit easier to create a branch other than master for a pull request. That way you can have your local master branch reflect the main master branch, and make changes only to your pull request branch as desired
• Assuming you do the former git rebase -i master is your friend for cleaning up duplicate commits. In this case, you can just fixup all subsequent commits into the first one.

Congrats on your first PRAW contribution, and thanks again!