allow custom history fetching

[revati]

Concrete use-case: pagination. There might be other use cases as well.

For more feature proof solution part after querying also should be extracted to separate function that can be overwritten. Its outside of scope for my current task.

[narrowtux]

The query has already been extracted into its own function by #65.

Maybe the part that applies the diffs from the list of changes in the current history function should be extracted so it can be used by others in combination with history_query in more flexible ways. I personally don't like the style with the callback.