Move meek's URL and front configuration into bridge_prefs.js
Since the tor in the 4.0-alpha branch (current tor-browser-bundle.git master) supports setting parameters in bridge lines, we can move some of the configuration out of torrc.
This is going to be good for supporting multiple meek backends (like the upcoming CloudFront). Each one can be defined in its own bridge line, instead of requiring a ClientTransportPlugin line with a distinct method name.
On the other hand, if you're planning to do any meek bundles based on maint-3.6, you don't want this, because it won't work with tor 0.2.4.