Live updating for statically-referenced assets

[mbostock]

Consider the following HTML:

<img src="test.svg">

If you replace the file test.svg, it’d be nice if this immediately updated in the client. We should be able to do this in conjunction with content-based hashing #260 since we already statically detect such file references.

The same should apply to data loaders: if test.svg is generated by test.svg.js, then saving a change to test.svg.js should re-run the data loader, and if the output is different, it should have a new content hash which then triggers an update in the browser.

[mbostock]

Fixed in #843. Duplicate of #908.