Add longest common sub-sequence algorithm

[piyawutpattamanon]

Hi, due to my work require me to write calculate "similarity" of strings, I've created a dynamic programming version of my own and I would like to contribute to pythainlp.

I will submit a pull request within 2-3 days if there is not any problem.

Reference
https://en.wikipedia.org/wiki/Longest_common_subsequence_problem

[bact]

Maybe we can put this somewhere inside pythainlp.util module :)