Add `chruby` plugin

Depends on `chruby-fish`: https://github.com/JeanMertz/chruby-fish

set `CHRUBY_AUTO_ENABLED` to `false` to disable auto loading Ruby versions on
directory change.

set `CHRUBY_ROOT` to point to the root path of chruby. The path will be
appended by `share/chruby/chruby.fish` and `share/chruby/auto.fish`.
Merge pull request #163 from JeanMertz/plugin-chruby