Command to migrate a concession group

[thekaveman]

Building on the API from #46 and the existing groups command (i.e. take into account filtering).

Usage

$ littlepay groups [-f GROUP_TERMS] migrate

Acceptance criteria

• A new sub command for groups is defined called migrate (or similar)
• The new sub command builds on the groups filter, migrating group(s) that match the given filter if any
• The new sub command uses the API endpoint implemented in #46 to do the migration
• Tests are written to cover the new sub command