Add support for opt reference

[jadamon42]

It would be really nice to add support for the opt reference, i.e.:
${opt:stage}

I think the only way to really do this would be to allow users to pass a cliOptions object to readYamlSync. Then you could treat it very similarly to the env reference.

Ideally, the following would be possible:

options: ${opt:}
.......
field: ${self:options.stage}

If the serverless file uses the opt reference, everything breaks, which is very unfortunate.

[jlitowitz]

I created PR #9 which should handle this, as I also came across this need. Along the way though, I found out you can work around this by passing in a context object into readYamlSync as follows. Just build up your desired opt values within the switch statement.

  return parser.readYamlSync(file,null,{
    opt: (x) => {
      switch(x[0]) {
        case 'stage':
          return 'dev';
        default:
          return x[1]; // expand upon this as necessary for your use case, ex:  x[1] || x[2] || x[3]
      }
    }
  });