Add option to keep both changes

[douglascamata]

Sometimes git is drunk and we need to keep both local and upstream changes. It would be nice to have a button for this.

[laughedelic]

Duplicate of #185?
Also there is some progress in #200.

[smashwilson]

👋 Thanks for taking the time to file an issue here! Unfortunately, this repository is being deprecated, so I won't be getting to this one here.

This one should be taken care of by the new git and GitHub package for Atom. We have "ours then theirs" and "theirs then ours" options in a drop-down menu on the conflict itself, so hopefully it'll be a bit more discoverable.