Add ability to specify env variables (#161)

Update yargs dependency to enable the new functionality.

bin/client

@@ -4,10 +4,11 @@ var open_url = require('openurl');
 
 var argv = require('yargs')
     .usage('Usage: $0 --port [num] <options>')
+    .env(true)
     .option('h', {
         alias: 'host',
         describe: 'Upstream server providing forwarding',
-        default: 'https://localtunnel.me'
+        default: 'https://localtunnel.me',
     })
     .option('s', {
         alias: 'subdomain',