Support associating an issue with a milestone based on the milestone name

Currently, associating an issue with a milestone happens based on the milestone's number:

$ ghi open --help
    -M, --milestone <n>              associate with milestone

This is inconvenient as it usually requires the user to find the milestone number first, and while possible, it is not obvious how to do this via the GitHub interface. It is of course also possible to look up the number via ghi milestone, but this too may not be obvious to many users.

Better would be to support associating an issue with a milestone based on the milestone's name, which the user usually already has in mind, and is in any case easy to remember. #62 already proposed this same improvement, but was closed as the OP never suggested but never actually submitted a pull request. I'm opening a new issue here to see whether the ghi team would be willing to implement this support on their own.