Make meek-client-torbrowser take the firefox command as a parameter

meek-client-torbrowser hardcodes the firefox binary and profile paths: linux mac windows. The problem is that when Tor Browser is reorganized, as it was in #11641 (closed), you need to make the corresponding change in meek-client-torbrowser, for example 178572f5.

meek-client-torbrowser already takes the full meek-client command on the command line; it looks like:

ClientTransportPlugin meek exec ./TorBrowser/Tor/PluggableTransports/meek-client-torbrowser -- ./TorBrowser/Tor/PluggableTransports/meek-client --url=https://meek-reflect.appspot.com/ --front=www.google.com

I don't know of a good clear way to encode two separate command lines into the command line of another program, except maybe to do them both as long strings and then parse the strings before calling exec.