Add regex support to whitelist

[fstanis]

This would allow using the same whitelist as used with uncss. Would also be good if we could add ignore as an alias of whitelist in options.

Example usage:

var posthtml = require('posthtml');
var uglify = require('posthtml-uglify');

posthtml()
  .use(uglify({ whitelist: [/\.[ab]ar/] }))
  .process('<style>#foo { color: red } .bar { color: blue } .aar { color: aqua }</style><div id="foo" class="bar">baz</div>')
  .then(function(result) {
    console.log(result.html); //=> '<style>#xz { color: red } .bar { color: blue } .aar { color: aqua }</style><div id="xz" class="bar">baz</div>'
  });

[fstanis]

Just realized there's already a PR for this: #28

@devinus any thoughts?