Option to disable Kochiku merges at the Repository level + rename auto merge

This builds off of #41 which made me realize that "automerge" is a poor name because it does not adequately convey what the feature does. Newcomers to Kochiku are likely to be confused by it.

In this PR I've removed all references to "auto merge" in most cases using `merge_on_success` at the build level or `allows_kochiku_merge` at the repository level.

If anyone has a better name than `GitMergeExecutor` feel free to suggest it. I definitely ran into the naming is hard problem with that one.

@square/kochiku-contributors @indspenceable