Add arguments to ignore importing comments, milestones, or labels
Prompt for username/password if none is provided in config or args

Now it will act more like trying to push commits to GitHub via HTTP.

This is more secure than having to store the password somewhere.
Prompt before importing issues

The script will still read from the source repository, but will not
write to the target repository until the user has accepted.

This will prevent the user from accidentally running the script twice,
or helping the user notice if they forgot to specify the issue ids to
import (preventing them from accidentally importing all issues instead).
Read known milestones and labels from target directory instead

Oops, I was reading them from the wrong place. I didn't notice this
sooner, since every milestone I imported had a milestone with the same
id already in the existing repository.

It's been fixed now.