Add program for playfair cipher in python

[illseption]

Resolves Issue #218

Description

Added an implementation of playfair ciper in python to systems/cryptography/playfair-cipher/python.

Technical Specifications

This program is written in python3.6.5. A python interpreter is required to run this program. The interpreter comes pre-installed in linux.

Screenshots

How to run

1. Open terminal
2. Change directory to the directory where the program is stored
3. Since this program is written in python3, run using "python3.version playfair_c.py"
4. If python3 is set to default, run using "python playfair_c.py"

Checklist

• [x ] Code compiles correctly.
• [x ] Changes are tested properly.
• [x ] Added the README / documentation, if necessary.
• [x ] Followed the Contributing guidelines.
• [x ] Resolved merge conflicts, if any.