Enhance text preprocessing functionality

[bdewilde]

Description

• moved existing text preprocessing functionality from a top-level preprocess module into a preprocessing sub-package, and reorganized it a bit
• add new functionality
  • replace_hashtags() to replace hashtags like #FollowFriday or #spacyIRL2019 with _TAG_
  • replace_user_handles() to replace user handles like @bjdewilde or @spacy_io with _USER_
  • normalize_hyphenated_words() to join hyphenated words back together, like antici- pation => anticipation
  • normalize_quotation_marks() to replace "fancy" quotation marks with simple ascii equivalents, like “the god particle” => "the god particle"
• changed a couple functions for clarity and consistency
  • replace_currency_symbols() now replaces all dedicated ascii and unicode currency symbols with _CUR_, rather than just a subset thereof, and no longer provides for replacement with the corresponding currency code (like $ => USD)
  • remove_punct() now has a fast (bool) kwarg rather than method (str) because it's easier and clarifies the difference between the two options
• removed some bad/awkward functionality
  • normalize_contractions(): this was a clunky, slow, and very limited attempt; better to use a separate, dedicated package
  • preprocess_text(): this was an awkward attempt at user convenience; better to let users mix and match their preprocessing pipeline as needed
• added more and better tests for all of the above

Motivation and Context

This part of the code base has acquired some cobwebs over the years, and Issue #250 reminded me that more work was required than a hotfix.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation, and I have updated it accordingly.