not support --napa_binding_binary_host_mirror

[smildlzj]

hi，
my .npmrc as fllow

napa_binding_binary_host_mirror=http://xxx.com/mirrors/xxxxxxxxxxxx/

but，when npm install，it can't read this setting.
so i read the source of node-pre-gyp, in lib/util/versioning.js

var host = process.env['npm_config_' + opts.module_name + '_binary_host_mirror'] || package_json.binary.host;

in napajs's package.json, this module_name is napa-binding, but in process.env is 'napa_binding'

so it cant override package.json mirror setting