Added RainbowParenthesesActivate command into plugin file

I would suggest adding RainbowParenthesesActivate. It's useful when you set filetype autocmd like...

````````````vim
autocmd Filetype scheme RainbowParenthesesActivate
````````````

It's better than RainbowParenthesesToggle action because it doesn't cause unintended "toggle off" when opening new buffer with :split.