Allow giving type of the bumping in the config

Should fix https://github.com/dylang/grunt-prompt/issues/19 for example.

I.e. 
```js
{
  bump: {
    options: {
      type: grunt.config('bump.increment'),
      createTag: grunt.config('bump.tag'),
      push: grunt.config('bump.push'),
    }
  }
}
```
@eddiemonge Fixed.