text: Title: Deprecate `return_str` parameter in `NLTKWordTokenizer` and `TreebankWordTokenizer`
  Body: Hello!
  
  I'd like to discuss a potential enhancements of `NLTKWordTokenizer` and `TreebankWordTokenizer`. For those unaware, the former is the tokenizer that is most frequently used, and is used in the `word_tokenize` function. It's also based on the latter class: `TreebankWordTokenizer`.
  
  An example usage as can be found in the documentation:
  
  ```python
  >>> from nltk.tokenize import NLTKWordTokenizer
  >>> s = '''Good muffins cost $3.88 (roughly 3,36 euros)\nin New York.  Please buy me\ntwo of them.\nThanks.'''
  >>> NLTKWordTokenizer().tokenize(s)
  ['Good', 'muffins', 'cost', '$', '3.88', '(', 'roughly', '3,36',
  'euros', ')', 'in', 'New', 'York.', 'Please', 'buy', 'me', 'two',
  'of', 'them.', 'Thanks', '.']
  >>> NLTKWordTokenizer().tokenize(s, convert_parentheses=True)
  ['Good', 'muffins', 'cost', '$', '3.88', '-LRB-', 'roughly', '3,36',
  'euros', '-RRB-', 'in', 'New', 'York.', 'Please', 'buy', 'me', 'two',
  'of', 'them.', 'Thanks', '.']
  >>> NLTKWordTokenizer().tokenize(s, return_str=True)
  ' Good muffins cost  $ 3.88  ( roughly 3,36 euros ) \nin New York.  Please buy me\ntwo of them.\nThanks  .  '
  ```
  
  ### The enhancement
  As you can see from the example, if `return_str` is True, then the `tokenize` method returns a space-separated string. However, the number of spaces is very inconsistent. Perhaps we would be better off stripping spaces on the ends, and replacing all sequences of multiple spaces with just one space.
  
  E.g. 
  ```python
  >>> NLTKWordTokenizer().tokenize(s, return_str=True)
  'Good muffins cost $ 3.88 ( roughly 3,36 euros ) \nin New York. Please buy me\ntwo of them.\nThanks .'
  ``` 
  instead of 
  ```python
  >>> NLTKWordTokenizer().tokenize(s, return_str=True)
  ' Good muffins cost  $ 3.88  ( roughly 3,36 euros ) \nin New York.  Please buy me\ntwo of them.\nThanks  .  '
  ```
  
  I figured I would create an issue for this to find out if others agree with this idea, before I put in the time to make this change for no reason. So, I'd like to hear your thoughts.
  
  - Tom Aarsen