I find the showindex example to be pretty useful on occasion, but
there were are couple of output tweaks I wanted, plus I wanted the
ability to specify a path to an actual index file instead of having
to open the whole repository. This makes those changes and expands
the example slightly.
This improves docs in some of the public header files, cleans
up and improves some of the example code, and fixes a couple
of pedantic warnings in places.