Add comments handler for merge & label

[parkr]

I'd love to have jekyllbot start to label & merge things.

Only members of @jekyll/core should be able to merge.

Label determines section to add to changelog.

Add label like:

@jekyllbot: merge +minor

Maybe SMS confirm? Nah.

[parkr]

Began this work here: 7915104

[parkr]

IT APPEARS TO WORK! jekyll/jekyll#4330 (comment)

Last piece is to ensure the reference isn't already in there. Whoops.

[parkr]

Hmmmmm looks like I already did that: https://github.com/parkr/auto-reply/blob/master/comments/merge_and_label.go#L238-L243

🎊