Add multi-character mapping support.

Prerequisite scaffolding for "quick makrs" feature.

Test plan:

* Added various mappings like "abcd", "cdef", "dekg", verified that they
  execute appropriately when something like "bcdekg" is typed.
* Console logging.

Note: I'm newbie to Vimium codebase and used Coffescript for the first time just today, so please feel free to point out any problems, bugs, style inconsistencies etc.