use of appdirs package, should fix issue #870

Hi! (First of all, thanks for working on Mailpile, it looks really good, I'm looking forward to it becoming a little more stable so I can start recommending it to my non-technical friends!)

I saw issue #870 and thought it was a nice small task to start with contributing, as I don't have extensive experience with larger python projects. If you can suggest other smaller python-based issues I'm also happy to work on that.

Question: the AppDirs optionally has an `appauthor` field, which may be used on windows machines for the user data directory - what the official author name of Mailpile? "Mailpile-Team"?