teach `uru admin add` to honor --tag

[Jon]

• Allow overriding of automatic tag name generation which is useful when testing multiple patch scenarios of the same ruby version
• Will likely require adding a new field to the persisted JSON ruby registry
• Will increase complexity as all tag name matching behavior will need to get smarter. This breaks the explicit tag naming linkage with ruby --version info

[Jon]

Refactor ruby tag impl. Fixes #18, fixes #21

Tag values used to identify a specific ruby will be string hash values rather than strings generated from parsing ruby --version. Tags will not be user visable, but will provide one level of indirection to a user visable TagLabel. This scheme will allow multiple flavors of the same ruby version to coexist and enable tag value customization by the user.

→ <<cset a3192adbbf63>>