Making installable package out of `paperlint.py`

[vdmitriyev]

Hi @misc0110

An installable package has been created out of your amazing paperlint.py.
If you like the changes, I am glad to create a pull request with those changes.
Despite introduced changes, the script should still be runnable as follows (no installation required, only cloned git repository):

python -m paperlint <file.tex/path> [-i/x <include/exclude switch>] [--error]

Change are here: https://github.com/vdmitriyev/Paper-Linter

P.S. Writing this as an issues, because not quite sure, if such structural changes are desired.