Support being passed a path

[mikepurvis]

Right now, the roslint_python and roslint_cpp macros support being passed lists of source files, or nothing, where the "nothing" behaviour is to glob and lint everything in the package. It would be great to be able to specify a path to lint everything below, eg:

roslint_cpp(DIRECTORY include)

Part of this could be supporting an optional corresponding FILES mode, which would be the same as the current behaviour for loose arguments, so the following would be equivalent:

roslint_cpp(FILES src/main.cpp src/foo.cpp)
roslint_cpp(src/main.cpp src/foo.cpp)

In this scenario, the follow calls would also be equivalent:

roslint_cpp(DIRECTORY .)
roslint_cpp()