Create URI::GlobalID scheme

Addresses https://github.com/rails/globalid/issues/6
Ah, okay. Is the same true for `'foo@bar'` and `'foo:99'`?

As a rule, inserting something into the middle of a complex string & then parsing the result, seems less strict than explicitly checking the validity of the new value in isolation. But maybe here the new parser is sufficiently strict to cover us?