Make the label optional

[jgrumboe]

Hi,

Thanks for this extension, super cool!
But it's failing in my case, because we don't use a dependencies label for PRs.

could not add label: 'dependencies' not found

Could you make the label optional or selectable per --label for example? So I could use it with an empty string as input.

Thanks,
Johannes

[rnorth]

@jgrumboe sorry for the slow response. I think that's a good idea, and I'd happily accept a PR if anybody wants to do this before I get around to it.

[jgrumboe]

#14 would fix my issue.
@rnorth would you merge it?