Improve tab completion in command mode

Pressing tab when all suggestions start with the same substring
will insert that substring (this is how bash autocompletion works).

cmd/micro/util_test.go

@@ -1,6 +1,9 @@
 package main
 
-import "testing"
+import (
+	"fmt"
+	"testing"
+)
 
 func TestNumOccurences(t *testing.T) {
 	var tests = []struct {