[Feature request] Customer the MAIN_URL from ENV

[wilianto]

Context

I have a use case where I want to use google i18n address + adding custom country area, for ex: "Other". In that case, I would like to setup my own mirroring address. Other benefit also, if https://chromium-i18n.appspot.com is down, we can change it to other mirror repository easily without a need to upgrade the lib version.

Proposal

Reading MAIN_URL from env will give the client flexibility to choose the repository address.