Add MIPS build

[shaman79]

In my case it was useful to run the proxy on router (TP-Link Archer C7) that has an MIPS processor.
I made the build by myself and tested that it is running OK. I think it might be worth including it in the standard releases.

Tested buidflags are GOOS=linux GOARCH=mips GOMIPS=softfloat

Thank you for this awesome tool.