Better handle changes to Skill settings definitions

[krisgesling]

If the settingsmeta of a Skill is modified in any way it is considered a new version of those settings and is blank for all users.

Hence anytime a Skill gets a minor update, all users have their settings wiped for that Skill. (Technically they aren't wiped, the settings still exist for the old settingsmeta but for the user it's effectively the same as it being wiped). An example of this is the News Skill. We cannot add new stations to the settings without every user needing to reselect their default station.
See: MycroftAI/skill-npr-news#96

Need to discuss the best way to handle this. Eg can we grab the last known settings and attempt to apply those values to the new settings if all the keys match?