Load config file in the Config class

[richteer]

Simple PR to move the config loading logic into the Config class itself. Since this class should be relatively self-contained, it seemed to be more appropriate there than in Halibot.

I kept Halibot._load_config() and Halibot._write_config() as a simple redirect. Halibot should still control when to read/write the config, and also should be controller of the current working directory. Thus, workdir is an optional parameter that is automatically filled by the stub functions on the Halibot class, but required for any direct calls to Halibot.config._load_config().

[coveralls]

Coverage decreased (-0.2%) to 93.061% when pulling 118cc24 on config-load-in-Config into d940184 on master.