Tag metrics

[kirillt]

Tag metric is a function which enables us to order tags. Technically, it can be alphabetical ordering but it isn't really useful conceptually. More convenient metric is popularity of a tag, which stands for amount of resources labeled with the tag.

With linear selector we can use only 1 metric. But with cloud selector we can use up to 3 metrics (see #7).

Possible metrics to implement:

• popularity — amount of resources standing behind a tag
• freshness — the latest date of modification among all resources standing behind a tag
• coupling — amount of common resources a tag has with other tags
• branching — amount of tags crossing given tag

It also should be possible to invert ordering by any metric.