Fix relative path issue #293

Added option 'basepath' so that long filenames in the reports can be avoided. For example, by passing '/foo/bar/' as basepath, paths like '/foo/bar/src/index.js' will be transformed into 'src/index.js'.
Here.   Because of `reporter_options.shortnames`, the filename no longer has the full path, but only the file's name.  Getting the dirname of it probably doesn't make sense anymore.