Add new encoding: playfair #37

Description

The Playfair cipher is a symmetric encryption method based on polygram substitution using grids. The Playfair cipher is a symmetric encryption method using polygram substitution using bigrams (pairs of letters), invented in 1854 by Charles Wheatstone, but popularized by his friend Lord Playfair, hence its name.

Reference

https://www.dcode.fr/playfair-cipher