Now with hybrid HTML/SVG tree drawing

This release introduces a form of tree-drawing that uses a mix of HTML (for positioning) and SVG (for line-drawing). It currently has a number of limitations compared to svgling.core, the biggest of which is that branching can be at most binary. The advantage is that it allows arbitrary HTML/MathJax in nodes. (My most immediate target for this mode of tree drawing, rather than just rendering syntax trees per se, is programmatic use for visualization of semantic composition in https://github.com/rawlins/lambda-notebook.)