Add NLTK

[kklemon]

https://www.nltk.org/

[ufoym]

Thank you for your contribution! Could you provide a set of common deep-learning-based toolkits for natural language processing and add scripts for building images like deepo:all-nlp-py36-cu90?

deepo/scripts/make-gen-docker.py

Lines 68 to 72 in bc37059

	# all modules with jupyter
	for pyver in pyvers:
	modules = candidate_modules + ['python==%s' % pyver, 'onnx', 'jupyter']
	postfix = 'all-jupyter-py%s' % pyver.replace('.', '')
	f.write(get_command(modules, postfix, cuda_ver, cudnn_ver))

[kklemon]

I can, but please also consider my thoughts in #59 regarding the implementation of too many standard pip or apt packages.