Add html coverage to makefile
Add html coverage to makefile

The destination is the coverage folder, since this was already
part of the gitignore.

Running make coverage still gives the text coverage
running make coverage type=html will build the html coverage

PHPBenelux/opencfp,localheinz/opencfp,localheinz/opencfp,opencfp/opencfp,GrUSP/opencfp,PHPBenelux/opencfp,GrUSP/opencfp,PHPBenelux/opencfp,GrUSP/opencfp,opencfp/opencfp,opencfp/opencfp,GrUSP/opencfp,PHPBenelux/opencfp,localheinz/opencfp,localheinz/opencfp