Add warning if several shortcut are used or disallow multiples shortcut

[MoOx]

According to specs/mailing list, using several shortcuts is not the same result as using transform.

Ref postcss/postcss#190 (comment)

A nice thing will be (in order to provide a safe "w3c" plugin) to disallow multiple shortcut usage or add a warning if several shortcuts are used at the same time.

poke @iamvdo

[jonathantneal]

Thank you, @MoOx. I have added a note and would like to add warnings as you suggest also, in order to provide the safest possible plugin.