Add a merge helper subcommand

# Summary

In the as-yet-unreleased `abc upgrade` feature, we're going to be outputting merge conflicts into the user's template destination directory. We could add subcommands to make it easy to launch an editor to resolve these conflicts, like `abc resolve outputs.tf`.

# Example

For example, suppose there's a template that outputs a file named "outputs.tf" that initially contains:

```
output "foo" {
value = a.b
}
```

Then suppose the user locally edits the file to be:

```
output "foo" {
value = x.y
}
```

While at the same time a new version of the template is released that contains:

```
output "foo" {
value = j.k
}
```

At upgrade time, abc detects this as an "edit-edit conflict" and asks the user to resolve it manually.

# Implementation of `resolve` subcomamnd

One way this could work: `abc resolve outputs.tf` would look at the filesystem to determine what kind of merge conflict it is, then launch vimdiff as apprioriate. For an edit-edit conflict involving `outputs.tf`, we could exec vim with three editor windows using `vim -d outputs.tf.abcmerge_locally_edited outputs.tf outputs.tf.abcmerge_from_new_template`.

# Possible extras
- Populate the middle pane of vimdiff with the lines that are the same between the two files. Probably requires integrating with a diff library.
- Use meld on Linux and $SOMETHING_GRAPHICAL on macos.
- Let users define their own editor, like how git uses $GIT_EDITOR
- A "resolve all" mode that scans for unresolved merge conflicts and presents each one in the editor
- A "--theirs" and "--ours" flags, analogous to git, to blindly keep one of the two versions