Add ability for overcommit configs to inherit settings

[tmandke]

No description provided.

[sds]

Appreciate the pull request, @tmandke.

An inheritance feature would be useful, but a straight up Hash#merge call is perhaps too simple. We would likely want to implement some form of deep merge, or at least consider what other tools in the space do (e.g. RuboCop).

Furthermore, some tests that assert the desired merge behavior should be included if we'd like to see this merged. Thank you!