[linkify] Allow editing rules

[ahal]

Currently if you create a rule but mess up the regex, there's no way to edit. You have to first delete it and then re-create it again. It would be a bit more convenient if you could run:

!admin linkify edit <rule> <re> <sub>

or perhaps:

!admin linkify new --force <rule> <re> <sub>

to bypass the existence check.

Then again, deleting a rule isn't terribly hard, so this may be a WONTFIX.