Add IssuesRepo class to gather repo issues

Uses the JsonRepo class to store data from the GitHub repo issues
API[1]. They toplevel issue, comments and events are stored.

The option --include-wikis has been renamed to --full-backup to save us
from an explosion of content selection options. Instead, the option is
code backup or code + wiki + issues + ... backup.

1. https://developer.github.com/v3/issues/