cppcheck: handle --diff-filter option

Add the possibility to the cppcheck script to pass git's
`--diff-filter` option.
Also improve comments a bit.
Travis: let cppcheck fail for new files

Change Travis configuration for cppcheck to fail on new files and keep on
warning about existing ones.
Merge pull request #1840 from LudwigOrtmann/cppcheck-selecta

Travis + cppcheck: let cppcheck fail for new files