Extend the commands API with More Key Options

[Patrick Dark]

The commands API needs more key options, including the (forward) 
slash/solidus character.

I've just tested the API and verified that use of it can restore my 
extension to 100% of its previous functionality if the aforementioned 
key is made available.

More specifically, my tab duplication extension uses the keyboard 
shortcut Alt+/, the choice of which has figurative significance. When I 
converted it from an XUL/XPCOM extension to a WebExtension, in order to 
keep the shortcut choice intact I had to use content scripts with 
document.defaultView.addEventListener("keyup"). This means that my 
extension can no longer duplicate tabs without a file:, ftp:, http:, or 
https: URL (due to content script match pattern restrictions) and 
therefore suffers from an inconsistent UI since it will duplicate some 
tabs, but inexplicably not duplicate others.