change launch intent / make configurable

[nealrs]

Right now, launchRequest assumes the intent will be named "Default Welcome Intent" which, sure, may be DF suggests, but isn't necessarily true for someone like me (my welcome intent is 'welcome.json') but how about making this configurable or grepping for 'welcome' somewhere in the action package?

public launchRequest(): ActionRequest {
  this.requestType = RequestType.LAUNCH_REQUEST;
  return this.intentRequest("Default Welcome Intent");
}