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!
Make CursorColumn the same as CursorLine

- These two highlight groups are the same by default, should keep
them that way.

- The motivation behind this is to make the scheme more compatible
with the vim-paren-crosshairs plugin