Implement builtins "visisted-dirs" and "jump-dir".
Implement builtins "visisted-dirs" and "jump-dir".

This addresses issue #27.

CodyGuo/elvish,elves/elvish,CodyGuo/elvish,elves/elvish,beni55/elvish,elves/elvish,ke7ofi/elvish,elves/elvish,ke7ofi/elvish,beni55/elvish,elves/elvish