feat: add update-notifier to warn when using an old version of rdme (#…

…435)

To test this out, you can replace the line in bin/rdme with the following:

```
updateNotifier({ pkg: { name: 'rdme', version: '1.0.0' }, updateCheckInterval: 0 }).notify();
```

Example nabbed from here:
https://github.com/yeoman/update-notifier/blob/HEAD/example.js

bin/rdme

@@ -1,6 +1,11 @@
 #! /usr/bin/env node
 const chalk = require('chalk');
 
+const updateNotifier = require('update-notifier');
+const pkg = require('../package.json');
+
+updateNotifier({ pkg }).notify();
+
 require('../src')(process.argv.slice(2))
   .then(msg => {
     // eslint-disable-next-line no-console