yaml support

[jdorfman]

I wonder if it would make sense to have support for yaml config's. That way you could do:

$ rump -bookmark local
$ rump -b prod

_config.yml:

bookmark:
  local:
    from: 'redis://127.0.0.1:6379/1'
    to: 'redis://127.0.0.1:6379/2'
  prod:
    from: 'redis://production.cache.amazonaws.com:6379/1'
    to: 'redis://127.0.0.1:6379/1'

[nixtrace]

Sure,

Let's keep it in mind if we'll add more flags.

For now, since we just have two flags, maybe using shell variables might be enough in dynamic envs. :)