readline: completion support

[felipensp]

Usage:

import readline

fn main() {
	list := ['mem', 'memory', 'method']


	mut r := readline.Readline{
		// completion_list: list
		// or   
		completion_callback: fn [list] (prefix string) []string {
			return list.filter(it.starts_with(prefix))
		}
	}

	dump(r.read_line('prompt> ')!)
}

[spytheman]

Excellent work, done very quickly.
Hats off to you @felipensp 🙇🏻‍♂️ .