Add Mish to the list of activation functions

[LifeIsStrange]

This should be trivial to make and would help Mish to become popular.

Mish consistently beat the old state of the art (swish) which consistently beat ReLU https://arxiv.org/abs/1908.08681v1

[dcato98]

I'll second that it would be nice to have the Mish activation (and Sine too!) in the official playground. In the meantime, feel free to check out my fork here which includes both Sine and Mish activation functions.