5.1.0 Improved merge conflict resolution

When a conflict happens:

1. A list of the conflicting files will be displayed
2. If configured (via "editor" property) the conflicting files will be opened in the user's preferred editor
3. When resolved, the user can return to the backport terminal. The user does not have to stage (git add) or commit their changes - backport will do this for them when they agree to proceed.
4. The backport process will continue as usual: the branch will be pushed and a PR created.

Added in #170