docs: add a very minimal --help flag

Unfortunately, nconf only provides usage for arguments specifically
configured through the argv stores. Arguments converted from other
stores are not displayed. Since we don't configure any specific argument
for the command-line, no help is displayed.

Fix #54.

lib/config.js

@@ -2,10 +2,13 @@
 
 var nconf = require('nconf'),
     path = require('path'),
-    net = require('net');
+    net = require('net'),
+    version = require('../package.json').version;
 
 nconf
-  .argv()
+  .argv({}, 'Usage: $0\n\nSee http://dashkiosk.readthedocs.io/en/v'
+        + version
+        + '/configuration.html for more information.')
   .env('__')
   .defaults({
     configuration: './config.json',
@@ -63,4 +66,8 @@ nconf
          }
        });
 
+if (nconf.get('help') || nconf.get('h')) {
+  process.exit(nconf.stores.argv.showHelp());
+}
+
 module.exports = nconf;