Tidy 'imports' in viewer.js

Also figure out how to disable a control 'properly' using D3.
Allow SplitsBrowser to be used 'offline'

By 'offline', this means when using local files in the browser.  In this
situation, AJAX calls cannot be made.

Essentially, this change boils down to extracting a function that reads
the data out of a string, and making that part of the public API.
Update version number and generated JS files.