Unnecessary dependency matplotlib is required to call ContextualWordEmbsAug

[shuntaroy]

Textual augmenters do not have to import audio stuff, don't they?
It's not so much bother to import matplotlib even when my project does not require any visualisation, but it sucks.
Though this seems not urgent, hope it's fixed in future.

from nlpaug.augmenter.word import ContextualWordEmbsAug
  File "/path/to/virtualenv/lib/python3.7/site-packages/nlpaug/__init__.py", line 2, in <module>
    from nlpaug.base_augmenter import *
  File "/path/to/virtualenv/lib/python3.7/site-packages/nlpaug/base_augmenter.py", line 5, in <module>
    from nlpaug.util import Action, Method, WarningException, WarningName, WarningCode, WarningMessage
  File "/path/to/virtualenv/lib/python3.7/site-packages/nlpaug/util/__init__.py", line 7, in <module>
    from nlpaug.util.audio import *
  File "/path/to/virtualenv/lib/python3.7/site-packages/nlpaug/util/audio/__init__.py", line 2, in <module>
    from nlpaug.util.audio.visualizer import *
  File "/path/to/virtualenv/lib/python3.7/site-packages/nlpaug/util/audio/visualizer.py", line 7, in <module>
    import matplotlib.pyplot as plt
ModuleNotFoundError: No module named 'matplotlib'

[makcedward]

Will fix it in next release