Make config file optional

[benmuth]

This fixes #29.

Problems this solves:

• users have control over if/how they want to use the config file
• config file path is customizable
• certain config file options weren't being used at all (for example, the verbose setting was always overwritten by the CLI)

Hopefully this doesn't break any use-cases/workflows. Let me know if there's anything you're skeptical about.