Add 'append and prepend' wrapper

Hi ; 

at sharypic we needed the 'append and prepend' style of the bootstrap (see : http://twitter.github.com/bootstrap/base-css.html#forms or a small screen shot http://cl.ly/1B451f1v3x311k193u1i) 

So i updated your generator, hope it helps.
Best, Martin.

PS: if something is messing let me know ;-)