Trigger updates in inheriting repos when "base" config is changed

[mattwynne]

New Feature

Here's the scenario:

• You have a "base" configuration in, say, your .github repo, and you have inherited that configuration in several other repos using the _extends: keyword.
• You decide you want to change the settings of all your repos, so you update the base settings.yml

Unfortunately, at least based on my observations, the bot is not yet clever enough to detect that the inheriting repos need to be refreshed when their inherited configuration changes.

It would be wonderful if, when a settings.yml file changed, we could scan other repos for any _extends: references to the file that changed, and update them too.

[mattwynne]

Ref: #179, #95

[rogerluan]

I decided to go with https://github.com/github/safe-settings, much better UX and supports many more features overall

[rogerluan]

Oh and it's from /github 😉