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
make npm-contributors a multi task

in order to run this task multiple times with different configurations
make commit count available in filter

In order to allow filtering/sorting based on the commit count.