Submodule support

Feature request: It'd be handy to have submodules be automatically inited and updated. (Perhaps only with a flag passed?)

Start by tracing the clone and pull paths described for ghsync, then inspect how command-line options are defined. Determine where Git submodules would be initialized and updated, and define the flag or default behavior before implementing it. Done means synced repositories handle submodules consistently and the chosen behavior is covered by the project's available checks.

Written by the indexing model from the issue text.