feat(include): include non-json files as strings

Add the following default options:
```javascript
includeFiles: {
    json: { resultType: "json"},
    html: { resultType: "string", separator: ""  },
    csv: { resultType: "string", separator: ";"  }
}
```
Add tests for the new options.

Only files defined in includeFiles will be included. There are no checks for binary files. The result type `array` is not yet supported.

Is that acceptable? Tell me if I can help further.