add linkify option and tests

- add linkify option, same as in `helper-license`
- add test for these cases - copied from `helper-license`, lol

#### Example:
Copyright (c) 2014 [Jon Schlinkert](https://github.com/jonschlinkert)  

works only if `linkify: true` and author is object and have `author.url` or `author.twitter`
thanks for doing this