Add filtering callbacks

[robertpainsi]

Add new filtering callbacks

• beforeFilter
  • Called before actually executing the filter logic. Returning false should abort filtering and not further callbacks should be called.
• filterItem
  • Called for each item. Returning a truthy value will consider the item in the filtering process and add it to the Result.allItems. This can be useful if you want to add custom filtering logic, like if you also filter by the title of the item.
• filter
• afterFilter
  • Called after executing the filter logic.

[robertpainsi]

For now only filterItem will be implemented. It's the only callback which users can't implement themselves. All others can easily be implemented without callbacks. However, other callbacks will be added after version 1.0.0.

[robertpainsi]

Using the library in real case scenarios, more before and after callbacks are needed to simplify using the library.