The existing `s:CRAndNohlsearch()` function doesn't work as-is in visual
or operator-pending modes. Add special handling for these modes that
results in the <CR> command mode mapping behaving differently in order
to support a selection.

TODO: Need to handle `cg/` `cg?`. Currently this inserts
`hlsearch<CR>gv`.

Make operator-pending versions of the `g/` and `g?` mappings.

Don't define the default `g/` and `g?` mappings if there are already
Nohai mappings or if those are already defined.

Add a `g:nohai_no_maps` variable to disable plugin mappings.

In addition to the default, don't load the plugin if the `CmdlineEnter`
and `CmdlineLeave` autocmd events don't exist.

Learned that these events were introduced in Vim 8.0.1206:
https://vi.stackexchange.com/questions/15875/how-to-test-if-certain-event-is-supported

Seemed like I should make the name more explicit.

* The `s:Nohai()` function is unnecessary, as it does the same thing as
  `s:AddMapping()`. Call `s:AddMapping()` directly instead.
* Don't bother checking `cmdwin_char`. Because we limit the
  `CmdlineEnter` `autocmd` to `/` and `?`, the `s:AddMapping()` function
  will only ever be called on `/` and `?`, thus checking its value in a
  condition is redundant.