Remove blanket spell checking from vimrc

Blanket spell checking looks very ugly on windows gvim. It looks even
worse on a windows console. This patch removes blanket spell checking
from windows files and attaches it to specific file types that should
always be spell checked (for now this is C and C++ source files).
Merge 'master' into 'base16'

This merge brings in some fixes for spell checking, enabling it only for
certain file types.