refactor/fallback_skills_v2

[JarbasAl]

make fallback skills use the bus like all other skills

companion to OpenVoiceOS/ovos-core#307

relevant discussion: MycroftAI/mycroft-core#2806

class OVOSFallbackSkill(OVOSSkill):
    """
    Fallbacks come into play when no skill matches an intent.  
    
    Fallback handlers are called in an order determined the 
    priority provided when the skill is registered.

    Handlers with the numerically lowest priority are invoked first.
    Multiple fallbacks can exist at the same priority, but no order is
    guaranteed.

    A Fallback can either observe or consume an utterance. A consumed
    utterance will not be see by any other Fallback handlers.
    
    A skill might register several handlers, the lowest priority will be reported to core
    If a skill is selected by core then all handlers are checked by 
    their priority until one can handle the utterance
    
    A skill may return False in the can_answer method to request 
    that core does not execute it's fallback handlers
    """

    def can_answer(self, utterances, lang):
        """Check if the skill can answer the particular question.


        Arguments:
            utterances (list): list of possible transcriptions to parse
            lang (str) : lang code
        Returns:
            (bool) True if skill can handle the query
        """
        return len(self._fallback_handlers) > 0


    ```

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[NeonDaniel]

Looks good to me