Acorn parser does not support import attributes

[timfish]

The older import assertion syntax works:

import coolFile from './something.json' assert { type: 'json' }

But acorn does not appear to support the newer import attribute syntax and it will not get added until Stage 4:

import coolFile from './something.json' with { type: 'json' }

PR #104 actually works around this issue by falling back to the parent loader when parsing fails.