readline: turn emitKeys into a streaming parser

In certain environments escape sequences could be splitted into
multiple chunks. For example, when user presses left arrow,
`\x1b[D` sequence could appear as two keypresses (`\x1b` + `[D`).

PR-URL: https://github.com/iojs/io.js/pull/1601
Fixes: https://github.com/iojs/io.js/issues/1403


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>