Closes #52.

To do:

• Ensure that subclassing works!
  • Subclasses of parsable need to be decorated with @parsable as well in order to guarantee that parsing them works
  • A subclass of a parsable that is also decorated with @parsable should fall back to the parents' name_decoder and other @parsable-specific params if the subclass doesn't specify them
    • How should this work with multiple inheritance?
    • Maybe don't inherit @parsable params?
• Use typing.assert_type() in tests?
• Write docs
  • Document mypy plugin
• Update changelog
• Rewrite long description
• Change the package's short description to no longer mention the "argparse-like-ness"
• Deprecate the old argparse-like parser