`contentType` file header

Hiya,

I added the possibility to use a `contentType` header in your files to allow overriding the default content types of 'application/json' etc.

E.g.:

```xml
//! contentType: "application/vnd.custom+xml"
<xml>
	<created>1</created>
</xml>
```

The same restrictions as with `statusCode` apply:

1. must be on the first line
1. comma-separated to declare multiple headers

Hope you like it, I sure can use it.