New feature request: execute `repoman get` in parallel

[stennie]

Commands are currently explicitly exec'd in series: https://github.com/basti1302/repoman/blob/69bae2da6bb5f36e00347c22c2b36cfca5e61dec/lib/repoman.js#L255

I suspect that may be to ensure multiple tasks execute in order for each repo, although it seems like command batches could be async per repo.

[basti1302]

I think keeping them in series should still be the default for most commands. Executing in parallel will at the very least make the output hard to read I guess, because output for different repos might be mixed up. But allowing for parallel execution as opt-in would probably be possible.