specify binary download path

[niyan-ly]

I was behind some mask that can not download pre-compiled binaries directly. So, may i ask if it is possible to set a env variaable like electron-mirror to specify the download url. Or if there is any way can make it.

[nekator]

This should be implemented in electron-builder. I started to work on a pr for this.
The PR should be ready in about a week.