Shadowsocksx-ng: Support using custom PAC file

[yukixz]

I'm using a whitelist PAC(default to proxy).
Seems the Auto mode support gfwlist rules and default to direct connection(blacklist).

Could you support using a PAC file when selecting Auto mode?

[yukixz]

I replace ~/.ShadowsocksX-NG/gfwlist.js with my custom PAC and set it immutable to meet my need. But it's a hacky way.

[lonelykid]

That is a dirty trick. I also want the feature of custom pac file given that Mac doesn't have a convenient way to switch proxy and it won't accept local pac file. I have to run a web server for now. This seems awkward.