Git configuration should be backwards compatible

The current style of check configuration is a hard-coded list of checks, or we fall back to a hard-coded list of defaults in code:

    git config pre-commit.checks "foo, bar"

or

    DEFAULT_CHECKS = [:foo, :bar]

With the new style of configuration https://github.com/jish/pre-commit/pull/107 the semantics have changed. We need to make sure that:

* People with old configuration are not disrupted.
* People with old configuration do not experience errors.