Add support for source maps

[paulmedynski]

Would you consider adding support for source maps? This would allow build tools that include a c-preprocessor step to accurately map line numbers back to the original source files during downstream compilation and unit testing. Imagine this flow:

0. Write some TypeScript code in several files.
1. Run build:
   a) Strip some code with c-preprocessor.
   b) Compile post-processed code.
   c) Package everything into a single file.
2. Run tests and errors occur on line X of package file.
3. Apply source map generated during Asking for advice on if/ifdef/ifndef and such #1 to map the errors back to source code lines in #0.

But the stripping in 1a) altered the source code, so the source maps generated by b) and c) don't line up with the originals.

For my purposes, having c-preprocessor take an existing source map as an additional arg to the processing step, and then altering it accordingly as it processes the contents, would be ideal.