Support 'fillchars'

Since I don't use the `fillchars` options (I have no chars separating vertical
splits, I find the bg highlighting to be enough), I set the same bg and fg
colors for the VertSplit group. This commit fixes this by highlighting VertSplit
with the same bg as SignColumn and LineNr and with a slightly lighter fg.

These ideas come from #10 from @wedgwood. Thanks!