Change how Snowflake handles client arguments

@richard just pointed out on IRC that the way Snowflake's client-side arguments are passed to the executable make them difficult to dynamically change through Tor Browser's preferences. For Snowflake, these are specified through the ClientTransportPlugin torrc option in the torrc-defaults file:

## obfs4proxy configuration
ClientTransportPlugin meek_lite,obfs2,obfs3,obfs4,scramblesuit exec ./TorBrowser/Tor/PluggableTransports/obfs4proxy

## snowflake configuration
ClientTransportPlugin snowflake exec ./TorBrowser/Tor/PluggableTransports/snowflake-client -url https://snowflake-broker.torproject.net.global.prod.fastly.net/ -front cdn.sstatic.net -ice stun:stun.l.google.com:19302,stun:stun.voip.blackberry.com:3478,stun:stun.altar.com.pl:3478,stun:stun.antisip.com:3478,stun:stun.bluesip.net:3478,stun:stun.dus.net:3478,stun:stun.epygi.com:3478,stun:stun.sonetel.com:3478,stun:stun.sonetel.net:3478,stun:stun.stunprotocol.org:3478,stun:stun.uls.co.za:3478,stun:stun.voipgate.com:3478,stun:stun.voys.nl:3478

Bridge lines, on the other hand, are specified in a seperate torrc file. See the built-in preferences for obfs4 and snowflake bridges.

Right now the only way to make changes to Snowflake client-side options (which have a huge impact on censorship) is to ship a new verison of Tor Browser or tell users to manually modify their torrc files.

@dcf also mentioned in !50 (closed) that we need to reconsider command-line options for Snowflake with the addition of new rendezvous methods. This is a related concern and we should make sure that how we chose to move forward works well with this scenario.

One option would be to instead specify command-line arguments through the pluggable transport specification PT args (as obfs4 does with the cert and iat-mode arguments). I haven't tried this, so I'm not sure it would work if two different bridge lines have the same fingerprint, but I believe it would allow us to specify multiple Snowflake configurations as separate bridges:

Bridge snowflake 192.0.2.3:1 2B280B23E1107BB62ABFC40DDCC8824814F80A72 url=https://snowflake-broker.torproject.net.global.prod.fastly.net/ front=cdn.sstatic.net ice=stun:stun.l.google.com:19302

Bridge snowflake 192.0.2.3:2 2B280B23E1107BB62ABFC40DDCC8824814F80A72 ampcache=https://cdn.ampproject.org/ ice=stun:stun.l.google.com:19302