Add function that returns a list of all functions in their respective category - #10

This is a resubmission of the previous pull request. Here is the description:

This new function in pygorithm.sorting and pygorithm.search returns a list of all functions in pygorithm.sorting and pygorithm.search.

Example usage:

>>> from pygorithm.sorting import modules
>>> modules()
['bubble_sort', 'bucket_sort', 'counting_sort', 'heap_sort', 'insertion_sort', 'merge_sort', 'quick_sort', 'selection_sort', 'shell_sort']

This will make it easier for users to see what functions are available to them without looking at the entire source code or documentation.