Add sourceMapIn option #827

babel/grunt-babel#14 suggests using grunt-contrib-concat to concatenate multiple files before compiling with Babel.

However, then the outputted source maps reference the file that was created by concat, not the original files. It would be awesome if Babel could take in an existing source map while it does its transforms, so that the outputted source maps reference the original files rather than an intermediate file.

• Many CSS-processor libraries deal with this by looking in a standard place for a map file (when loading style.css, look for original source maps at style.css.map).
• Uglify deals with this using a sourceMapIn option.