[FEATURE] Add requirements_dev.txt

[Andrew Glick]

Problem Statement

Issue #3 saw the removal of development dependencies from the requirements.txt file. I agree with this decision, however, I do not agree with limiting their reference to only the README.

Proposed Solution

Instead, development dependencies should be added to a requirements_dev.txt file.

James Murphy from the YouTube channel mCoding did a great video about packages that goes over this very concept (and others). See how he adds the requirements_dev.txt here: https://youtu.be/DhUpxWjOhME?t=515.

[Cameron Urban]

@Antyos Thanks for the suggestion, this seems like a good idea! I'll work on implementing it in the next release.

[Cameron Urban]

This was fixed with the latest commit!