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.