Linked assets should auto-refresh on edit

[mbostock]

A generalization of #415. For example if you have an <img> tag, we detect this a linked file; we could generate a URL with a hash, and push down the changes over the socket if the file changes.

[trebor]

#905 explicitly covers stylesheets and images. other types i can think off the top of my head are:

• fonts
• audio
• video
• scripts with the script tag
• iframe for local html files

i can image fully general code that looks for href or src fields starting with the asset file name - rather than handling them one by one.

[mbostock]

This may end up colliding with #843 which affects nearly everything involving files and imports.

[trebor]

got it, maybe hold of until #843 lands.

[mbostock]

Fixed in #843.