Add `preserveHacks` interface

This closes #4.

USAGE:

    var csswring = require('csswring');
    csswring.preserveHacks = true;
    csswring.wring(css);

This preserve some CSS hacks:

  - `_prop: value;` (IE6)
  - `*prop: value;` (IE6-7)
  - `prop/**/: value;` (except IE6)
Reset option in test
Update README
Replace \t
Merge pull request #5 from hail2u/preserve-hacks

Preserve hacks