v3 features

Here’s the stuff I’d like to land in the next major version:

Consolidate the require.js support into a single transform that uses the require.js optimizer to do all the work
Fix the very complex situation with base assets. This will remove a lot of complexity and means that we cannot support behavior: url(relative/path/to/foo.htc), AlphaImageLoader(src='relative/path/to/foo.bar') and GETSTATICURL('relative/path/to/foo.bar') anymore.
Web worker support
Disallow relative hrefs from HTML fragments and get rid of the stupid fixBaseAssetsOfUnresolvedOutgoingRelationsFromHtmlFragments and assumeThatAllHtmlFragmentAssetsWithoutIncomingRelationsAreNotTemplates transforms.
Make relations fully managed by their source asset. This could simplify the API and will get rid of the sometimes huge assetGraph._relations array. Won’t have to break assetGraph.findRelations, I think.
Come up with a replacement for GETSTATICURL based on a system.js plugin.
Webpack support.
Get rid of all transpilation.
Remove the support for angular.js templates.
Model '...'.toString('url') as a relation and map GETSTATICURL('...') to that, but with a deprecation warning (https://github.com/assetgraph/assetgraph/pull/569).
Add crossorigin getter to relations (https://github.com/assetgraph/assetgraph/issues/578)

This is all leading up to adding support for incremental builds in some shape or form, which will presumably also require a major version bump. That’ll take a while to get right, though, so perhaps we should consider that a v4 thing for now.

// cc @Munter @gustavnikolaj @guybedford