Add builtin proxy settings

[Yuan-Quan]

Problem Internet connection in China is kind of broken, and VPN software seems can't catch the traffic from Java programs. Result in the installer almost unusable in China. Several friends of mine have complained that they can't install impact.

Describe the solution you'd like Add an option to use proxy.

A ways I can get it working in China Use cfw-tap virtual adapter to route all traffic into VPN software.

Additional context Console output:

$ java -jar ./ImpactInstaller-0.9.4.jar
                   ...
DOWNLOADING https://github.com/ImpactDevelopment/ImpactReleases/releases/download/4.9.1-1.12.2/Impact-4.9.1-1.12.2.json
java.net.SocketTimeoutException: Read timed out
        at java.net.SocketInputStream.socketRead0(Native Method)
        at java.
        ...
Trying some hacks to get this to load!
Undoing hacks!
java.lang.NullPointerException
        at java.util.Hashtable.put(Unknown Source)
        at java.util......
        ...

[chinese-wzq]

It took me over 6 hours to try again and again at least 80 times.We chinese really need the function.