Support Manual Merge

[davidrusu]

It could be useful to allow applications to manually handle a merge. An example would be a conflicting change to an apps critical config value. The app may prompt the user for a decision on which value to choose.

DoD:

• app's can indicate to gitdb that it wants to handle merges for a key
• app's are notified of a need to handle a conflict
• app's can resolve a conflict