Make insert_local work for an array of local files.

I made a minor change that has `#insert_local` work with both single local files and arrays of local files.

All tests pass, but cane currently seems to be worrying about the ABC metric of this new code; the new `#insert_local` method has an ABC complexity of 18. I don't know if this is really an issue, so I didn't try to alter my implementation or touch the `--abc-max` value.

If there's any issues with this PR, I'll be glad to fix them. I'd like to able to use a multi-file `#insert_local` as soon as possible.