Allow custom completers to be dynamic

... Dynamic in the sense that the completion func should be provided with the _readline up to the cursor_ (modulo command name)

I'm currently unable to port my code to v2 because completers aren't provided with enough context work dynamically.
```
$ ls /
bin tmp sbin etc
```
`/` isn't passed to my custom Cmd.Completer until the user hits the space bar. Instead the arguments passed to my completer are empty.

It'll be pretty awesome if the following should also work as expected:
```
$ ls / --color=auto
bin tmp sbin etc
```