Added contextual menu option

Added ability to enable and disable the extension through the
contextual menu accessed by right clicking.
Added features for disabling and enabling popups

There is now a contextual menu option to enable Rikaitan. Also added an
event listener so that if the extension is active, there will be an
option to enable or disable popups. Also changed the hide popup key
from esc to h, and made the functionality visible to users in the mini
help menu.
Merge pull request #5 from jordenhill/Menu-option-and-hide-popup-key

Menu option and hide popup key. Added functionality relating to enabling and disabling popups.