Add options to set location for CA file

[Kiarashz]

When setting download location with --download option, https URLs with self-signed certificates can not be accessed failing with the following error.

 prebuild-install WARN install unable to verify the first certificate

The code seems not to respect NPM configuration for cafile and does not provide its own option too.

[vweevers]

PR is welcome. I'm assuming it would be a few lines of code, reading something like process.env.npm_config_cafile and passing it to the request.