Add support for annotations that highlight only a subset of a line

[Luro02]

For IntelliGrade, it would be nice to be able to highlight only parts of a line.

This would be useful when you have multiple annotations in the same line. For example, here:

int four = 4;

We could have an annotation for the magic number 4 and one for the name which is a bad identifier.
Currently the entire line is highlighted, because artemis4j doesn't pass the autograder source positions to intelligrade.

I don't think artemis supports annotations that only span parts of a line, but they could be converted into a line number here.