Add a option to choose default plugin path.

[ashen0xa]

Shadowsocks subscription usually specifies plugins like obfs-local. In standalone ss client they should be put in the same directory with the client executable, however, in this plugin where those plugins should go is unclear and causing a lot of confusion. While there is a per-connection option to set plugin path, in a subscription doing so is extremely tedious.

This patch add a plugin-wise option that will prepend itself to plugin when the per-connection plugin is not an absolute path. The implementation may be a little junky but it works on my machine(TM).

Fixes #4 #5.