Consider replacing `autoMerge` boolean with enum

[crenshaw-dev]

Was reading through k8s API conventions and came across this:

Think twice about bool fields. Many ideas start as boolean but eventually trend towards a small set of mutually exclusive options. Plan for future expansions by describing the policy options explicitly as a string type alias (e.g. TerminationMessagePolicy).

It's a good point. Should we consider something like mergePolicy, mergeStrategy, etc.? Maybe mergePolicy: OnCommitStatusPassed and mergePolicy: Manual.

[zachaller]

#199

Not exactly related but share the same spirit just in opposite direction. Maybe, the API naming should be consistent between them.