Added example context-menu Chrome extension

This example was taken directly from

https://developer.chrome.com/extensions/samples#search:contextmenus

and is pretty self-explanatory.
Added Chrome extension

Completes the first phase of development.

Features:
  - Select a command and right click, then click on "Lookup on
    explainshell.com" to jump to a new tab
  - Rudimentary icon
  - Put relevant documentation in the README