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.
Merge pull request #29 from phleet/phleet-sourcemap-header

Support specifying sourcemap via HTTP headers