Adding support for image tag

[ghost]

I have been studying your code and somehow managed to add a static image with hardcoded size (will generalise it later). The Problem I am facing is the height of the label doesn't increase after I have added the image. I have created a small demo if you want to have a look https://github.com/shwetsolanki/HTMLLabel

When I call sizeToFit method, the HTMLLabel resizes correctly.

Let me know your thoughts.

Thanks