use alfred-firefox workflow (if installed) to get the current URL from Firefox

[brttbndr]

The alfred-firefox workflow uses native messaging and a browser extension to integrate with Firefox. It provides a command-line client that talks to the plugin.

https://github.com/deanishe/alfred-firefox

This PR uses that client to get the current tab's URL from Firefox.

If the workflow is not installed, it falls back to using the existing behavior of "focus URL bar, copy text to clipboard"

I have tested the following:

• alfred-firefox not installed -- pinning a bookmark still works
• alfred-firefox is installed -- pinning a bookmark works
• alfred-firefox is installed -- with popular tags or check if bookmarked is enabled, pinning a bookmark now works (where previously it failed)

[spamwax]

Thanks, I'll merge this as soon as I can test it on my macOS. Looking at that workflow, I wish that Firefox had a better API and AppleScript support.