Add PHP and comment block support

This PR introduces PHP and comment block support. ```<txp:php>``` tags act as embedded PHP blocks  and you receive PHP completion and highlighting within the tags:

```
<txp:php>
    echo "I'm actually PHP, sucka!" . PHP_EOL;
</txp:php>
```

And ```<txp:hide>``` tags act as comment blocks as that is what they are:

```
<txp:hide>
    I'm just a bloody plain-text comment that documents
    <txp:variable name="banner_color" /> usage and I'm
    not visible on generated pages.
</txp:hide>
```