Make it possible to remap 'a' to another character.

The character 'a' is conflicted with the argument object of targets.vim, to me, the argument object is used more frequently than the after object, so I want to keep 'a' for the argument object and map another character (e.g. A) to after object.

Sadly, for now, I can only remap the argument object, because targets.vim provides a solution for this:

```vim
autocmd User targets#mappings#user call targets#mappings#extend({
\ 'A': {'argument': [{'o': '[([]', 'c': '[])]', 's': ','}]},
\ })
```

But what I want is the opposite.