Cool, I'm glad you guys like it
With respect to highlighting other brackets, there are two options:
* Highlight the inner most pair of brackets of any enabled type
* Highlight the closest pair of each type, so there would often be multiple highlights going on
Also, the current underline colour is a 50% blend of the caret colour and the background colour: I'll make the ratio configurable, as it makes a big difference to visibility.
String highlighting is an interesting one, as the strings are already highlighted via syntax highlighting, so I don't imagine you'd get terribly much by adding the underlines. Are there some cases I'm missing here?