feat(runner): support sourcemaps in spec files

This feature allows folks who are generating their spec files from a
different language to see stack traces that use the line numbers from
their sources before translation.

This commit introduces a dependency on the `source-map-support` library.

For general information about sourcemaps, refer:
-  http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/
-  https://github.com/evanw/node-source-map-support
-  https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/view