[FEATURE REQUEST] MyPy check-untyped-defs option.

[GuillaumeCrowdcomfort]

Feature Request:
The MyPy options section doesn't allow for the --check-untyped-defs option. I would love to be able to specify it.

Reason:
Some functions cannot be correctly typed, but the content can be. Forcing to ignore typing in functions is disabling the ability to catch errors based on types for part of code.

Solution:
Add the option to the list of valid options specified in the .prospector.yaml.