Mastadon support

[elad661]

This should be as simple as adding a new provider (like twitter.py and tumblr.py), importing it in bot.py, and adding it to the providers list in https://github.com/elad661/picdescbot/blob/master/bot.py#L100

Ideally the posts should be marked as sensitive by default, just in case picdescbot uploads something bad. The filtering is quite good at the moment and events like these are rare, but I wouldn't want to risk it.