Test Pilot & Web Extensions messages: prefValueRequest and addonListRequest

[groovecoder]

Now that #1022 is merged, and for mozilla/blok#12 (comment), I'd like to request some new message types between Test Pilot and other (Web Extension) add-ons:

When Test Pilot receives a message like {type: 'prefValueRequest', pref: '?'}, it should return a response message like {type: 'prefValueResponse', pref: '?', value: '<value>'}, so Web Extensions can query for Firefox pref values.

When Test Pilot receives a message like {type: 'addonListRequest'}, it should return a response message like {type: 'addonListResponse', addons: [<addonID1>, <addonID2>, ...] }, so Web Extensions can query Firefox for other installed add-ons.

[johngruen]

We're not going to do this, but we do have in product UI warning users about incompatible add-ons