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`.

plugins/chruby/README.md

+## 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`.

plugins/chruby/chruby.load

+#
+# 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`.
+#
+set -q CHRUBY_ROOT; or set CHRUBY_ROOT /usr/local
+
+if test -f "$CHRUBY_ROOT/share/chruby/chruby.fish"
+  . "$CHRUBY_ROOT/share/chruby/chruby.fish"
+else
+  echo '`chruby` plugin loaded but chruby-fish not installed.' \
+       'See: https://github.com/JeanMertz/chruby-fish'
+end
+
+if test "$CHRUBY_AUTO_ENABLED" != "false" -a \
+        -f "$CHRUBY_ROOT/share/chruby/auto.fish"
+
+  . "$CHRUBY_ROOT/share/chruby/auto.fish"
+end