Hide NonText after loading the colorscheme

I like to configure NonText characters to display as black, so that the
tildes that vim shows for blank lines don't show up. But by outsourcing
.vimrc to separate files (see commit 4a7d2c1), I messed up this
behavior. This commit fixes the problem by specifying highlighting in
solarized.vim, so that it is called after loading the colorscheme.