Initial implementation of livestreamer config files

It only supports global configuration for now instead of configuration
per plugin (needs to investigate how ConfigParser really works, and for
some motive livestreamer.set_plugin_option() isn't working), and doesn't
support both types from options that has 2 different types (I am only
supporting the str variation). But it seems to work (tested on both
Python 3.4 and 2.7).