Add merge command

[twpayne]

Fixes #224.

This PR adds a merge command that invokes merge.command to do a three-way merge between the destination state, the source state, and the target state.

merge.command defaults to vimdiff. You can set a different one in your config file, for example (in TOML):

[merge]
  command = "gvimdiff"
  args = ["-f"]

cc @markstos