Support Alt(Meta) key prefix
Refactor #89

This is what I really meant to do. i.e.

   type Keymap [2]map[termbox.Key]KeymapHandler

seemed too hard for the regular human being.
In this change it's

   type Keymap [ModMax]RawKeymap
   type RawKeymap map[termbox.Key]KeymapHandler
Kill unused var
Merge pull request #90 from peco/support-alt-prefix-redux

merge lestrrat's refactoring
Merge pull request #89 from peco/support-alt-prefix

Support Alt(Meta) key prefix