add option to delete uneeded CSS

Currently only deletes -moz-border-radius, -moz-box-shadow and
the webkit equivalents. Seems to reduce CSS by about 1% before
gzip and 0.5% after gzip. Not a great reduction, but possibly
might start making a bigger difference if I can come up with
sane regexes to delete ie6 & 7 specic rules and RTL only rules.