Resolve sourceMappingURLs relative to the JS file

[danvk]

If source-map-explorer finds a path to the map file in the JS file, it will try to read it relative to the current directory. Instead, it should read it relative to the JS file's directory:

$ tail -1 dist/dygraph-combined.js
//# sourceMappingURL=dygraph-combined.js.map

$ source-map-explorer dist/dygraph-combined.js
/Users/danvk/github/dygraphs/node_modules/source-map-explorer/node_modules/convert-source-map/index.js:31
    throw new Error('An error occurred while trying to read the map file at '
          ^
Error: An error occurred while trying to read the map file at dygraph-combined.js.map
Error: ENOENT, no such file or directory 'dygraph-combined.js.map'