Support PAC script generation.

chnroutes only works when using VPN with modified route
table. But it could became a more useful script.

This commit makes chnroute support the PAC file generation, as
a new "pac" platform. Now, HTTP/Sock4/Sock5 users could take benefits
of chnroutes, including users of SSH and Shadowsocks.

Signed-off-by: Tom Li <biergaizi@member.fsf.org>