Treat .c files as C headers

[denizzzka]

(Rare but useful case)

Standards isn't prohibit directive like #include "c_file.c". But if included file is .c or .i (preprocessed file) it will allways be treated as C++ header by dpp.

I planning to use this change to ease creating bindings by adding -save-temps flag into (almost any) C build environment and then processing obtained preprocessed files by dpp - so it helps to avoid of collecting of C compilation flags and passing all of them into dpp

[denizzzka]

.i files excluded because preprocessing of C++ files produces files with same extension

[denizzzka]

Test is coming soon