Use tag name or release url as input

Hi! I've been using this Github Action and I run into an issue:

The release must use `${{ github.ref }}` as its tag:

https://github.com/alexellis/upload-assets/blob/f055c93bb9c5d934b26573f22c020a3b86bf7372/src/get-release.js#L11

This may not be a great idea, as seen here: https://github.com/actions/create-release/issues/116.

I think there should be an option to specify either the tag name or the release url, like https://github.com/actions/upload-release-asset does. This would both avoid the mentioned issue and let the users pick their desired release tags.
Thank you!