The structure expected is:
<div class="columns">
<div class="column" width="40%">
contents...
</div>
<div class="column" width="60%">
contents...
</div>
</div>
Support has been added for beamer and all HTML slide formats.
Closes#1710.
Note: later we could add a more elegant way to create
this structure in Markdown than to use raw HTML div elements.
This would come for free with a "native div syntax" (#168).
Or we could devise something specific to slides
