Add optional `show` argument to `FreqDist.plot()` (#2786)
Add optional `show` argument to `FreqDist.plot()` (#2786)

* Add optional 'show' argument to 'FreqDist.plot()'

* Add optional 'show' argument to 'ConditionalFreqDist.plot()'

* Fixed small issue in conditions

* Added information on *args and **kwargs to plot() methods' docstrings

* Added percents option to non-cumulative plot() for FreqDist and ConditionalFreqDist

Also modified docstrings accordingly
nltk/nltk,nltk/nltk,nltk/nltk