Support specifying sourcemap via HTTP headers

As noted in
https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit,
the URL for sourcemaps can be specified via a 'SourceMap' header. It was
previously called 'X-SourceMap', so let's support that too.

I ran the provided tests, and all of them passed except the amd-test, which looks like it was already broken on master.

If you'd like a test for this functionality, I'm happy to provide one, though I'm not sure how you'd like it to look. Boot a node server with that header?