Feat: Selective commit

[ezrizhu]

We should allow the user to choose what they want to commit.
One way to do this is to list all the changes in the upperdir in a file, open up and editor so user can delete the files that they don't wish to commit, and then commit the rest. In the prompt file we could also describe the type of change, (modify/create or delete).

[mgree]

Feels closely related to #23---we should do them both as an overhaul of the commit logic.