Add support for raw html/script pages to be incorporated into the site.

Currently if you want to create a content page, you must do so as markdown. This patch set modifies the system to support php files on the server which will be executed on the server and the output of which will be injected into the client. In addition, any &lt;script&gt; tags defined will be evaluated in the main site context.

.php is used instead of .html for two reasons:
1) Support server side processing (if needed)
2) Many of the scripts are written to ignore/exclude .html files from the documentation and contribute directories

To see the above in action, place the following into the file documentation/99-Super_Balloon.php
```html
<?php
print "Sample balloon output from PHP:<br>".__FILE__."\n";
?>
<br>
<br>This sample has been:
<div id='fluxbox' style='border: 1px solid black;padding:0.5rem;'>
</div>
<script>
document.getElementById ('fluxbox').textContent = 'Scriptified';
</script>
```

You should then see 'Super Balloon' in the top-level menu. Loading that page should result in output telling you the full file path of the php script, as well as the dynamic client side script changes with the text 'Scriptified' appearing in the DIV element.