rbenv-use: fuzzy matching plugin for rbenv

These formulae allow an rbenv user to use fuzzy version matching when
switching Ruby versions.

    $ rbenv whatis 1.8.7
    1.8.7-p374
    $ rbenv whatis 2
    2.0.0-p247
    $ rbenv use 1.8.7
    $ rbenv version
    1.8.7-p374 (set by RBENV_VERSION environment variable)

Closes #21152.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>