Add relevance score for a custom tag taxonomy

When adding a tag on a post, I would like to allow the user to give the tag a score reflecting the relevance of that post in the specified tag.

For example: The user adds a new post which is mainly about "beds", however it is also relevant for "textile" (but not as relevant). Thus giving the "beds" tag a higher score than the "textile" tag. When fetching posts on the "textile" tag the post will not be placed as high up as on the "beds" tag archive.