Support for external url in sections ("index map")

[jkrems]

The spec describes an alternative for embedded the nested source map sections by providing the URL to the source map.[1] Currently source-map only supports map in sections.

See #16 for the original issue and #127 for the basic implementation of sections.

[1] https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit?pli=1#heading=h.535es3xeprgt