Allow ability to provide arbitrary command arguments to rsync

Instead of creating options for each rsync feature, this commit allows
the arbitrary use of an —-args="" parameter when using the rsync
deployment provider.

In future additional options may still be provided through the use of an
explicit beam option (via Symfony commands) but this change will
allow the flexibility without needing to foresee what options will be
desirable.
Make rsync —-args option use VALUE_REQUIRED mode

Just for correctness and appearance in the help:

    --args                Additional arguments for rsync

vs.

    --args                Additional arguments for rsync (default: "")
Merge pull request #40 from heyday/rsync-args

Allow ability to provide arbitrary command arguments to rsync