adds "noPrefixes" option, to opt-out vendor prefix check

Based upon https://github.com/Modernizr/Modernizr/issues/1082. By adding `noPrefixes` to the config options, a flag is set to empty the vendor prefixes array used by testAllProps.

Checkout https://github.com/Modernizr/Modernizr/issues/1082#issuecomment-32118659 for details.