implement sentence length distribution plot

[jgregoriods]

Resolves #89 . Documentation was changed accordingly. Errors in the documentation (IMDB file name and wrong argument name in plots) also fixed.

[meghdadFar]

@jgregoriods Thank you very much for contributing!

Since you are merging from a fork, I cannot run your code in a branch, and for some reason, I cannot see your latest changes when I clone your fork.

Could you please clone this repo, branch out a feature branch, include your changes there, and then create a PR from that branch?

My other point is about the docs. We change the documentation only in the directory sphinx-docs/. We then run sphinx-build -b html sphinx-docs/source docs/ so that Sphinx automatically generates or updates the docs/ directory. This directory is then used by GitHub to generate documentation pages. Note that you can put your images in sphinx-docs/figs and edit the docs in sphinx-docs/source.

Looking forward to merging your changes!