Build on Windows with MinGW

[cgrs]

There should be an alternative building method for those people who doesn't want to use VS.
I achieved the building with TDM-GCC (x64) on Windows 10 successfully, and I think this should be reflected on the build docs.

Basically it's the same method on building libsass with MinGW, but after building libsass, clone sassc into that directory, and run mingw32-make -C sassc.