Added target arch support
Added target arch support

Previously electron-prebuilt would download the prebuilt binaries
for the architecture native to the platform. However there are
cases where being able to download for another architecture is
favourable.
This patch adds support for setting the architecture through
npm's --arch= parameter. (Issue #53)

mafintosh/electron-prebuilt,electron-userland/electron-prebuilt,RnbWd/electron-bin,electron-archive/electron-prebuilt,brave/electron-prebuilt,airtimemedia/electron-prebuilt