Bug Formatting API

This pull request mostly moves code related to formatting bug reports from ```src/plugins/reporter-bugzilla.c``` to ```src/lib/problem_report.c```.

I attempted to get rid of usage of ```struct strbuf``` in favour of using ```FILE *``` in the public function interface. I had to commit a pair of minor tweaks to the functions taken from ```repoter-bugzilla.c``` because of this technical move.

Closes #303