Add ability to specify env variables (#161)

Update yargs dependency to enable the new functionality.

README.md

@@ -33,6 +33,12 @@ Below are some common arguments. See `lt --help` for additional arguments
 * `--subdomain` request a named subdomain on the localtunnel server (default is random characters)
 * `--local-host` proxy to a hostname other than localhost
 
+You may also specify arguments via env variables.  E.x.
+
+```
+PORT=3000 lt
+```
+
 ## API ##
 
 The localtunnel client is also usable through an API (for test integration, automation, etc)

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',

package.json

@@ -12,7 +12,7 @@
     "axios": "0.17.1",
     "debug": "2.6.8",
     "openurl": "1.1.1",
-    "yargs": "3.29.0"
+    "yargs": "6.6.0"
   },
   "devDependencies": {
     "mocha": "~1.17.0"