introduce livestreamer-cursesrc file

The (optional) livestreamer-cursesrc file holds configuration options.
There are 2 at the moment:
- LIVESTREAMER_COMMANDS: the list of different commands you want to be
  able to use to call livestreamer, cycle with L.
- DEFAULT_RESOLUTION: resolution for new streams, can be a str, dict or
  callable. See livestreamer-cursesrc.sample.
It is exec'ed as a python source file, so you can do whatever you like
in it.
update README, bump version