Improve [attr~="value"] selector (attribute contains word)

Improve [attr~="value"] selector (attribute contains word)

Instead of first splitting attribute value into words, and then
checking that one of words is equal to given value

   grep { $_ eq $value } split ' ', $attr;

use regexp anchored to words boundary for word matching

   /\b\Q$value\E\b/

Strictly speaking it makes [attr=~"value"] selector not conformant to
CSS2+ specification, but it shouldn't matter in real life.  See
http://www.w3.org/TR/CSS2/selector.html#attribute-selectors

  [att~=val]

     [..] If "val" contains white space, it will never represent anything
     (since the words are separated by spaces). If "val" is the empty
     string, it will never represent anything either.

Suggested-by: Matt S Trout <mst@shadowcat.co.uk>