Added Firefox signing without explicit id

[adamhwang]

As of Firefox 48 the "id" property is now optional:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings

Extensions published after FF48 will automatically have its id generated. web-ext sign supports this through an optional "--id" flag:
https://extensionworkshop.com/documentation/develop/getting-started-with-web-ext#packaging-your-extension

This PR adds support to sign and publish Firefox addons without an explicit id in its manifest. Instead, the id can be specified with the env var WEXT_SHIPIT_FIREFOX_EXTENSION_ID. This further standardizes the way shipit publishes web extensions across browsers and provides the ability to use the same webext for all three supported extension stores.

Working example: https://travis-ci.com/adamhwang/ita-matrix-powertools/jobs/288630882

[kamalbennani]

@LinusU Any news regarding this? thanks