Vim: add "guu" mapping to uncomment a line

I'm very used to The NERD Commenter, where I had separate mappings for
commenting (`<leader>cc`) and uncommenting (`<leader>cu`), so even
though I could use a single mapping now (`gcc`) for both commenting and
uncommenting, I find myself hitting `gu` to uncomment, and then
refraining my natural impulse to just hit `guu` to uncomment the current
line. (In practice I end up continuing to `guc` to uncomment the current
line, or `guac` to uncomment the current comment, single-line or
multi-line).

So, let's just set up a mapping for this basic case, so that `gcc`
comments and `guu` uncomments, which has a nice symmetry. In visual
mode, `gc` will comment and `gu` will uncomment.