use a config/init pattern

This reworks all the Assert configuration to follow a config/init pattern.  This allows for a clear precedence for specifying and overriding settings.  Settings are applied in 4 scopes in this order: User, Local, CLI, ENV.

This enables two new features: loading Local settings from a relative `~/.assert.rb` file and overriding settings using the CLI.  Both of those features will come after this new pattern is in place.

@jcredding this is a WIP.  Would you mind reading over the new README and make sure everything makes sense?  This is essentially my requirements doc for the config/init pattern updates.  Thanks man.