Load Privoxy as needed

[robot-dot-win]

Is your feature request related to a problem? Please describe.

ss_privoxy.exe will be loaded automatically as soon as Shadowsocks runs, but in fact it's only needed if system proxy is used. However some users never use system proxy - they use SwitchyOmega for browsers and Proxychains/Proxyfier for other applications, in order to get precise control.

Describe the solution you'd like

Load Privoxy only as needed, for reduction of resources.