Add autocomplete

Add easy syntax to support autocomplete. Current idea at the moment is

```nim
proc nameComplete(i: Interaction, current: string) {.async.} =
  discard

cmd.addSlash("ping") do (name {.complete: nameComplete.}: string) {.async.} =
  echo name
```
I think its a bit verbose though so might improve it once I play around with auto complete for a bit