Add in support for Mersenne Prime Twister PRNG

[wasade]

The current random number generator used is a linear congruential generator and is not a high quality random number generator. A brief discussion on random number generators can be found here with references to additional sources. If possible, it would be fantastic if seqtk relied on the Mersenne Twister, the source of which can be found here or alternatively, the algorithm can be used from the GNU Scientific Library.

[lh3]

I have wrapped MT-64 a while ago. I now put it in seqtk. Please try and let me if you have problems. I have not tested it myself.

[wasade]

Awesome, thanks! Was it not needed to update this call as well? Wasn't sure if that was intentional or not