[Proposal] Completion for the Ag command

- Category
- [ ] Question
- [ ] Bug
- [x] Suggestion

## Proposal

After thinking for a (long) while thinking about the problem of interactive search
with `:Ag`, after a [first proposal](https://github.com/junegunn/fzf/issues/1211) on how to implement it, I believe the way to support something like that is to implement a custom wildmenu completion for the command.

I think it fits the vim philosophy quite nicely, as it uses vim native features and works in the way a user would expect it to.

## POC implementation

Here's my first attempt to implement something like this.

```vim
fun! CompleteAg(A,L,P)
if a:A == ''
return a:A
endif
return system("ag -o " . shellescape('\b\w*' . a:A . '\w*\b') . ' | cut -d":" -f3- | sort -u')
endfun

command! -bang -complete=custom,CompleteAg -nargs=* Ag call fzf#vim#ag(, 0)
```

## Demo

Using the implementation above in the `fzf.vim` project

[](https://asciinema.org/a/Xfdw74uzRJyju9zWrJ10UVcOQ)

Thoughts?