Setting bufhidden=hide in vimrc

This is particularily valuable for various scripts that perform manipulation
on buffers, as unloading a buffer not only requires saving, but discards
modification history for undos.
Now using slatemg for terminal vim (not just gvim)

I have not yet committed my slate-based slatemg theme; previously, it did
not degrade well for 8-, 16-, or 256-bit colors, but it now does well.