Feature: Misskey/Calckey support

[ferdinandosimonetti]

Hi, I'd like to add support for Misskey/Calckey instances to Feediverse.
I've identified an already existing Python module that works with my Calckey instance.
Misskey/Calckey authentication/token generation is different from Mastodon's, it involves explicit browser usage by the user, and the resulting config auth section will be, of course, different.

EDIT I've added Misskey/Calckey support here, by:

• adding Misskey.py both as setup.py requirement and as imported feature on feediverse.py
• modified setup() function to take into account the different approach about token creation between Mastodon and Misskey/Calckey
• adding a boolean config item (ismisskey), see above
• modified main() to use Mastodon or Misskey-compatible approach to post