Git configuration should be backwards compatible

[mpapis]

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 #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.

[mpapis]

no tests for this one, tested it manually ... I guess we drop it anyway for 1.0.0 release.

[coveralls]

Coverage remained the same when pulling 2c88157 on bugfix/migrate_old_git_config into 831d8b2 on master.

[mpapis]

actually why not add tests :)

[coveralls]

Coverage remained the same when pulling 2116065 on bugfix/migrate_old_git_config into 831d8b2 on master.

[jish]

"actually why not add tests :)"

:)