Add implementation of Huffman Codes

[Greedy Algorithms](https://github.com/tirthasheshpatel/Data-Structures-and-Algorithms/tree/master/Greedy%20Algorithms) contains the file [huffman_codes.c](https://github.com/tirthasheshpatel/Data-Structures-and-Algorithms/blob/master/Greedy%20Algorithms/huffmann_codes.c) which is not implemented yet.
Go ahead and try to implement it!