Add AMAZON.FallbackIntent and handler to ask-cli-x branch

[habuma]

A lot of what has been added in the master branch in the past year or so is, IMO, unnecessary. But, I did enjoy seeing the addition of the AMAZON.FallbackIntent and would like to see it in the ask-cli-x branch for users of the new ASK CLI v2.0.x.

Without this intent and a corresponding handler, almost any utterance will match the HelloWorldIntent, even if the match is very low...making the list of sample utterances for HelloWorldIntent seem pointless. With the fallback intent in play, the utterances that match HelloWorldIntent make more sense and other utterances that don't make sense get captured by the fallback intent.

Moreover, the fallback intent is something that most Alexa skills will want to have in play, so including it in this template eliminates the need for the skill developer to add it themselves.