fix empty trailing line issue

In a case where the last contributor is being replaced by .mailmap
git shortlog contains an empty line at the end - we need to strip that.
make JSON key configurable

in order to add 'author' or 'maintainers' fields as described here:
https://www.npmjs.org/doc/files/package.json.html#people-fields-author-contributors
make filter configurable

In order to customize the list of contributors one now can
pass a custom filter function with the options.
Can also be `false` in order to disable the default filter.

fix #2