Header comments signal write override

[ryardley]

Default outputFilename is index.js whilst headers are checked to know whether or not to overwrite an index file.

The create-index package uses a specific string that acts as a command to work out if it is allowed to overwrite a file or now. I think this should be built in perhaps following the model laid down by eslint.

/* indexr-allow-overwrite */
//  This file is autogenerated by indexr.
//  Do not edit this file.
//  Check this file into source control.
...