obfsvpn: create new dialer from PT args

The dialer is meant as a way to avoid having to use or think about pluggable transport arguments (you can provide them all as typed configurations on the dialer), however, sometimes we already have the pluggable transport arguments in their parsed form (eg. when using a higher level protocol to negotiate the args) and need to construct a dialer from them. This patch allows constructing the dialer and getting the original arguments back out.

(this will be useful when proxying the connection as the socks5 package expects a pt.Args)