ENH: Clickable/tappable <a> links in dynamic text

[iccir]

Currently, SwiffHTMLToCoreTextConverter doesn't handle <a> elements.

Work to do:
• Add a new text attribute similar to SwiffTextVerticalOffsetAttributeName, this would contain an NSURL or NSString
• When SwiffHTMLToCoreTextConverter encounters <a>, add this attribute to the CFAttributedStringRef
• Ultimately pipe this up through SwiffPlacedDynamicText to SwiffView, which would create tappable/clickable hotspots.