feat(webdriver-manager): add seleniumPort command line option

Added seleniumPort command line option so that the standalone selenium server
can be started with the supplied port number as opposed to the default port 4444.
```
$ webdriver-manager start --seleniumPort 4443
```

bin/webdriver-manager

@@ -69,12 +69,13 @@ var binaries = {
 
 var cli = optimist.
     usage('Usage: webdriver-manager <command>\n' +
-        'Commands:\n' + 
+        'Commands:\n' +
         '  update: install or update selected binaries\n' +
         '  start: start up the selenium server\n' +
         '  status: list the current available drivers').
     describe('out_dir', 'Location to output/expect ').
-    default('out_dir', SELENIUM_DIR);
+    default('out_dir', SELENIUM_DIR).
+    describe('seleniumPort', 'Optional port for the selenium standalone server');
 
 for (bin in binaries) {
   cli.describe(bin, 'install or update ' + binaries[bin].name).
@@ -200,6 +201,9 @@ switch (argv._[0]) {
       process.exit(1);
     }
     var args = ['-jar', path.join(argv.out_dir, binaries.standalone.filename)];
+    if (argv.seleniumPort) {
+      args.push('-port', argv.seleniumPort);
+    }
     if (binaries.chrome.exists) {
       args.push('-Dwebdriver.chrome.driver=' +
           path.join(argv.out_dir, executableName('chromedriver')));