HTML export: Allow special colors for each TODO keyword

HTML export: Allow special colors for each TODO keyword

Wanrong Lin writes:

    Right now in the HTML export the TODO keywords have either
    class="todo", or class="done". That loses all the face properties
    in the original TODO keywords. I think the TODO keywords faces are
    important visual aids to differentiate different types of TODO
    items, so I just wonder whether it is possible to keep the faces
    in the HTML.

This makes sense.  This commit adds, to each TODO keyword, an
additional class named after the keyword.  For example:

  <span class="todo WAITING">WAITING</span>

So each todo keyword gets class "todo" or "done" depending on which
general type it is.  And in addition it gets itself as class.

So go to your CSS file and configure like this:

.todo { font-weight:bold; }
.done { font-weight:bold; }
.TODO { color:red; }
.WAITING { color:orange; }
.DONE { color:green; }

Thanks to Sebastian Rose for the multiple-classes trick.