Adds WriteAttributeTo method

The following was throwing an exception that WriteAttributeTo did not exist

```csharp
@helper test(string className)
{
    <p class="@className"></p>
}
```

The `WriteAttributeTo` method was missing from the `NancyRazorViewBase`
Merge pull request #1011 from lukesmith/razor-write-attributes-in-helper

Adds WriteAttributeTo method for razor helpers