add drawer state

This adds the `data-ur-toggler-component="drawer"` for sliding menus (we could also instead add a secondary state attribute on the data-ur-set, but the example doesn't have a data-ur-set). This was a flexbox alternative to using more JS to set a height/min-height on the body and change it whenever the window resizes or a toggler inside the drawer is clicked etc. The CSS can also be tweaked so that the drawer does not slide or has a set width.

Example HTML:
````html
<body data-ur-set="toggler">
<div id="wrapper" data-ur-toggler-component="content">
  <div id="menu" data-ur-toggler-component="drawer">
    ...
  </div>
  <div id="page">
    <header data-ur-toggler-component="button">Menu</header>
    ...
    <div id="mask" data-ur-toggler-component="button"></div>
  </div>
</div>
</body>
````

Example Sass:
````scss
#wrapper {
  display: -webkit-box !important;
  display: -webkit-flex !important;
  display: flex !important;
  overflow-x: hidden;
  &[data-ur-state="enabled"] {
    #menu {
      @include transform(translate3d(100%, 0, 0));
    }
    #page {
      @include transform(translate3d(50%, 0, 0));
    }
  }
}
#menu, #page {
  @include transform(translate3d(0, 0, 0));
  @include transition(0.3s);
  width: 0;
}

#menu {
  -webkit-box-flex: 1;
  -webkit-flex: 1;
  flex: 1;
  margin-left: -50%;
  &:not([data-ur-state="enabled"]) {
    height: 0;
  }
}
#page {
  -webkit-box-flex: 2;
  -webkit-flex: 2;
  flex: 2;
}

#mask {
  background: rgba(0, 0, 0, 0.3);
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  &:not([data-ur-state="enabled"]) {
    display: none;
  }
}
````