add fish shell support

completions/rbenv.fish

@@ -0,0 +1,22 @@
+function __fish_rbenv_needs_command
+  set cmd (commandline -opc)
+  if [ (count $cmd) -eq 1 -a $cmd[1] = 'rbenv' ]
+    return 0
+  end
+  return 1
+end
+
+function __fish_rbenv_using_command
+  set cmd (commandline -opc)
+  if [ (count $cmd) -gt 1 ]
+    if [ $argv[1] = $cmd[2] ]
+      return 0
+    end
+  end
+  return 1
+end
+
+complete -f -c rbenv -n '__fish_rbenv_needs_command' -a '(rbenv commands)'
+for cmd in (rbenv commands)
+  complete -f -c rbenv -n "__fish_rbenv_using_command $cmd" -a "(rbenv completions $cmd)"
+end