Added EPUB & MOBI to PDF converter

[malhotra-paritosh]

This is a Python script that converts .epub and .mobi format ebooks to PDF files for reading across devices.
It is a GUI-driven script (made using Tkinter). The user can select the file to be converted and it uses convertAPI to carry out the conversion

This PR has he complete Python script for the task, along with the required supplementary files (Readme.md etc)

Fixes #31

Type of change

Choosing one or more options from the following as per the nature of your Pull request.

• NOTE: These boxes can be checked using [X]

• Bug fix (non-breaking change which fixes an issue)

• [ X] New feature (non-breaking change which adds functionality)

• [ X] Documentation Update

Checklist:

Please tick all the boxes that are fulfilled by your Pull Request.

• [X ] I have named my files and folder, according to this project's guidelines.
• [X ] My code follows the style guidelines of this project.
• [ X] My Pull Request has a descriptive title. (not a vague title like Update index.md)
• [ X] I have commented on my code, particularly in hard-to-understand areas.
• [ X] I have created a helpful and easy to understand README.md, according to the given README_TEMPLATE.
• [ X] I have included a requirements.txt file (if external libraries are required.)
• [ X] My changes do not produce any warnings.
• [X ] I have added a working sample/screenshot of the script.

[AdityaJ7]

@all-contributors please add @malhotra-paritosh for code

[allcontributors]

@AdityaJ7

I've put up a pull request to add @malhotra-paritosh! 🎉