VS2010+: Add rudimentary support for CMAKE_RC_FLAGS.

Hi,

in my project (https://github.com/mity/mctrl) I need to pass option `-n` to resource compiler `rc.exe` when building with MSVC. Unfortunately the "Visual Studio 12 2013" generator seem to ignore `CMAKE_RC_FLAGS`, unlike Makefile-based ones.

Hence I added some rudimentary support for it. Note that it just maps `-n` to `NullTerminateStrings` in Project Files (for MSVC >= 12, because I do not have older installed).

All other (unknown) flags go to `AdditionalOptions` for MSVC >= 10 (for MSVC 10 and 11, this means all options).

I know the support is very incomplete and there is a lot of space for improvement. But hey, it solves my issue and I think it may be a good start for more complete support.

This is my 1st patch for CMake so I am very unfamiliar with its code base. Hence I tried not to be very intrusive and kept the patch small. Please be lenient :-)