Add option to define additional subtypes for tokens

This patch adds an option called `subtype`, which allows the
definition of additional CSS classes for token names. This can be
useful, to modify the style of the main token type for special cases
(e.g. a special type of string or comment).

I would like to spawn a discussion with this PR. I am not entirely sure if this feature is even useful. I am also open for suggestions for a different name for the option.
I would prefer something like:

```js
if (o.subtype) {
	var subtypes = _.util.type(o.subtype) === 'Array'? o.subtype : [o.subtype];
	Array.prototype.push.apply(env.classes, subtypes);
}
```

Also please use braces and semicolons per the coding guidelines of this project.