Commands with shell interface should support the Sh monad

[CGenie]

We currently have sshPairs which takes a list of text tuples. But actually any command that supports running subcommands, like su -c, bash -c, ssh should support the Sh monad so we can run the code more or less like this:

shelly $ do
    ssh "machine" $ do
        ...

This way we get full typechecked programs in the subcommand.

[gregwebs]

Its a good idea. I don't think this would be the Sh monad since the semantics are going to end up being different. A different monad could support a more limited API: it would be a lot of work to attempt to support the entire Shelly API running remotely.