Let the users have metadata (e.g. title) if they need it. Added an argument in WikiCorpus __init__() to specify if metadata is needed. Previously, it was set to False and could not be toggled.

Make Wikipedia corpus metadata accessible.

Allow users to access metadata by allowing self.metadata in WikiCorpus to be set by a parameter. However, Dictionary() raises "TypeError: decoding to str: need a bytes-like object, list found" if metadata is returned. So, introduced a dictionary_mode parameter in get_texts() so that metadata bypasses the dictionary, and goes directly to the user.