Auto merge of #979 - nathan-at-least:nathan.cleanup-nonofficial-tags, r=ebfull

Auto merge of #979 - nathan-at-least:nathan.cleanup-nonofficial-tags, r=ebfull

A script to remove "unofficial" tags from a remote, such as github.

Officialness is determined by a regular expression. ;-)

This is handy because we often want upstream Bitcoin tags for local
diffs, but sometimes we accidentally upload them to github which then
claims they are our releases, which is confusing and misleading.