Make withPrompts, withArguments, withOptions chainable

Close #631 
is this enough ?
```javascript
RunContext.prototype.withArguments = function (args) {
  this.args = this.args.concat(_.isString(args) ? args.split(' ') : args);
  return this;
};
```
or you prefer
```javacript
RunContext.prototype.withArguments = function (args) {
  var argsArray =  _.isString(args) ? args.split(' ') : args;
  assert(_.isArray(argsArray), 'args should be either a string separated by spaces or an array');
  this.args = this.args.concat(argsArray);
  return this;
};
```