allow dynamic server lists

[ceejayoz]

Our server IPs change as we scale up/down on AWS.

Ansible has a neat dynamic inventory capability that'd be nice to implement here as well.

[taylorotwell]

Feel free to make a pull request, but remember the file just compiles down to PHP. Meaning you can put plain PHP in the file and do whatever dynamic stuff you need to do before making the @servers call.

[bgetsug]

Works great with Hostbase, so I posted an example: https://github.com/shift31/hostbase-cli#laravel-envoy

w00t!