Implement input-chains for vim-emulation

[elkowar]

input chains are command mappings like gg, dw, etc that are composed of several, chained inputs. Some of these will have timeouts, i.e. mappings in insert mode (kj for escape is a typical example), whereas others are basically leader binds that shouldn't have a timeout. We should check if we can unify those or if they are conceptually different, and then implement both.