Make customizable

including / follow up PR #4 

I'd like to spread contributors into `author`, `maintainers` and `contributors` fields.

With this changes i'm able to add multiple tasks filtering multiple parts of the contributor list into different keys in my package.json

Example:
```js
'npm-contributors': {
  bower: {
    options: {
      file: 'bower.json',
      commitMessage: 'update bower authors',
      filter: false,
      as: 'authors'
    }
  },
  author: {
    options: {
      file: 'package.json',
      commit: false,
      filter: function(contributors) {
        return contributors[0].name;
      },
      as: 'author'
    }
  },
  maintainers: {
    options: {
      file: 'package.json',
      commit: false,
      filter: function(contributors) {
        return contributors.slice(1).filter(function(contributor) {
          return contributor.commitCount >= 10;
        });
      },
      as: 'maintainers'
    }
  },
  contributors: {
    options: {
      file: 'package.json',
      commitMessage: 'Update contributors',
      filter: function(contributors) {
        return contributors.slice(1).filter(function(contributor) {
          return contributor.commitCount < 10;
        });
      }
    }
  }
}
```