Improve :highlight command highlighting
- Use the same highlight groups for "default link" with and without
bang.
- Match some common line-continuation use.
- Match :hi clear variants.
- Highlight color-name values.
Resync vim.vim and generator/vim.vim.base.
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
