Add .overcommit.yml file

Change-Id: I4d9b38b90a73f15a46fe1aeaa5bbd29e049bf603
Use subject instead of let(:subject)

Correct a simple oversight. Using `subject` is more idiomatic.

Change-Id: I33e419946c4598d0e28d4bdb30f4ef0a1d57e8b1
Update Rubocop config to ignore more lints

This will reduce the noise.

Change-Id: If7c199ba8f022d876742029cc48ccd2b77c88e56
Change top-level hook types to use CamelCase

Previously, the hook type names were snake case (`pre_commit`, etc).

This was a bit annoying as it was difficult to remember to switch
between underscores for the group name and camel case for the hook name.

Convert the naming convention to use camel case. Note that individual
settings are still snake case to make them further distinct from the
other camel-cased entries.

Change-Id: I47f7dd83452de43684d9d01b43464ba49f468592