Add support for the textArea element from TinySVG 1.2.

&lt;textArea&gt; adds a width, height and text wrapping over a standard &lt;text&gt;. It is not part of the standard SVG spec; it belongs to TinySVG 1.2. As such it may or may not be appropriate for SVGKit. However it's simple to implement on top of SVGTextElement and is useful for my needs so I'm creating this pull request in case it's wanted.