Upper-case matched tag names to get expansion working
Prevent default behavior of captured click events
Remove outline around selected editor in demo page
Refactor HTML->Markdown and Markdown->HTML conversion functions

Rather than hard-coding a (case ...) or (cond ...) option for each
supported Markdown node type, every individual Markdown node type will
now be represented instead by an entry in a map of conversions. This
map will provide functions for transforming a Markdown node of the
corresponding type into an appropriate HTML node, and vice versa.