added proxy option for downloading the unicode file
added proxy option for downloading the unicode file

If we are behind a corporate proxy the `postinstall` script fails with connection refused so using the `process.env.HTTP_PROXY` setting we fix this by telling `http.get` to use the proxy server.

waleedsamy/node-unicodetable,owale/node-unicodetable,owale/node-unicodetable,waleedsamy/node-unicodetable,dodo/node-unicodetable,tdanecker/node-unicodetable