add async

[georgyo]

This is dependent on async being merged into rawlink (https://github.com/haesbaert/rawlink/pull/18) but I thought it would be good to get feedback early.

Since Async depends on Core and ppx_jane already, I used that in the command line parsing. It could just as easily use Async.Deferred.run_in_async_wait instead of the Command module to start the process. If there is desire to have the CLI interface be identical, I'm happy to make that change.