Create rawhtml.html

This allows to inject raw html into any content file. For example, my client uses an appointment booking service which requires a generated html snippet to be inserted into the content page. 

```
{{< rawhtml >}}
    <div id="etermin">
        <iframe ...>...</iframe>
        <script>...</script>
    </div>
{{< /rawhtml >}}
```

layouts/shortcodes/rawhtml.html

@@ -0,0 +1,2 @@
+<!-- raw html -->
+{{.Inner}}