Spec should identify a consistent cross-browser ID scheme for extensions

[mikepie1]

Particularly useful for site pages sending messages to extensions

[mikepie1]

Per meeting http://browserext.github.io/minutes/2017-02-02.html we discussed not having a cross-browser ID scheme that required registration/uniqueness in v1.0 spec. Site pages sending messages to extensions will be required to use a switch/case to use the proper ID, depending on the browser. The issue of random IDs is covered in #14