Add support for configuring INPUT_FILTER doxygen option

[peci1]

I need this option for a little transformation of my code so that Doxygen understands it better.

In particular, I need to set:

INPUT_FILTER           =  "sed 's/\([ <]\)::/\1/g'"

This removes the global namespace designator from all elements. I.e. ::std::string will become std::string. In the headers, I chose to designate all symbols with the global namespace (to avoid possible errors when someone does a using namespace), but Doxygen is very much confused by these, complaining a lot about unmatched function signatures that differ just in ::std::string vs std::string.