Add support HTTP/HTTPS/SOCKS proxy

[incendium]

It would be nice if MultiMC could support a proxy when launching Minecraft. Adding support for additional arguments when launching Java would also be an acceptable solution here.

You can specify a proxy in Java by setting any one of the following ENV variables:

http.proxyHost
http.proxyPort
https.proxyHost
https.proxyPort
socks.proxyHost
socks.proxyPort

You can specify ENV variables on the command line by prefixing the variable name with -D (e.g. -Dhttp.proxyHost=127.0.0.1).

[pantheis]

Let me second the above, but also add that MultiMC should itself support a system defined proxy under Windows and allow the user, via a configuration tab, to configure network settings to either use no proxy, the system proxy or a custom defined proxy. This missing feature is stopping me from being able to use MultiMC in some environments.