Custom Merge Hooks

A user thought it would be cool to be able to define custom stored procedures that trigger on different merge stages.

Custom stored procedures could be run at various stages of the merge like:

1. `dolt_one_merge()`
2. `dolt_conflict_detected()`
3. `dolt_schema_conflict_detected()`
4. `dolt_data_conflict_detected()`
5. `dolt_conflict_resolved()`
6. `dolt_schema_conflict_resolved()`
7. `dolt_data_conflict_resolved()`
8. `dolt_merge_committed()`
9. `dolt_merge_aborted()`

If a user defines logic in these procedures, that logic would be executed at the appropriate stage.