Being able to customize shell invocation

Currently the shell invocation interpreter that is invoked by an event is hard-coded at https://github.com/hashicorp/serf/blob/master/cmd/serf/command/agent/invoke.go#L51

It would be useful to be able to specify a different one, like `/bin/bash`.

Or just execute the file, and let the file determine what interpreter to use by using whatever shebang has been specified in the file itself.