add url updates for active file, with sidebyside mode too

the url here is

http://blockbuilder.org/micahstubbs/2a83e4d6cbd0536bdce126b82134a035#mode=sidebyside

it would be nice if you could link directly to a file, like this

http://blockbuilder.org/micahstubbs/2a83e4d6cbd0536bdce126b82134a035#sketch.js#mode=sidebyside


No file or test is named in the issue. Start by tracing how the browser reads the URL fragment and selects the active file, then verify that a direct file link also preserves sidebyside mode. Done means both example URL forms open the requested file and display the requested mode.

Written by the indexing model from the issue text.