Add more `stylish time` attributes to Firefox browser window

For users to make themes on specified dates, e.g Christmas, ~~Valentine's Day~~, April Fools, independence day, etc.

Example
```css
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
[stylish-month="12"][stylish-date="25"] :-moz-any(menu, menupopup, menuitem) {
  background: green !important;
  color: red !important;
}
```