Allow proxy credentials in `meek-client --helper`

meek-client doesn't allow a proxy username and password when using the browser helper. I think that at the time (circa Firefox 32) it wasn't supported using the API available to browser extensions.

But Firefox 45 added credentials support for SOCKS proxies, and while it's not as straightforward for http/https proxies, apparently what you can do is set a webRequest.onAuthRequired listener, and check inside it whether details.isProxy is set (since Firefox 60-ish?).