Add Mish Activation Function

Mish is a new novel activation function proposed in this [paper](https://arxiv.org/abs/1908.08681).
It has shown promising results so far and has been adopted in several packages including:

• [TensorFlow-Addons](https://github.com/tensorflow/addons/tree/master/tensorflow_addons/activations)
• [SpaCy (Tok2Vec Layer)](https://github.com/explosion/spaCy)
• [Thinc - SpaCy's official NLP based ML library](https://github.com/explosion/thinc/releases/tag/v7.3.0)
• [Echo AI](https://github.com/digantamisra98/Echo)
• [Eclipse's deeplearning4j](https://github.com/eclipse/deeplearning4j/issues/8417)
• [CNTKX - Extension of Microsoft's CNTK](https://github.com/delzac/cntkx)
• [FastAI-Dev](https://github.com/fastai/fastai_dev/blob/0f613ba3205990c83de9dba0c8798a9eec5452ce/dev/local/layers.py#L441)
• [Darknet](https://github.com/AlexeyAB/darknet/commit/bf8ea4183dc265ac17f7c9d939dc815269f0a213)
• [Yolov3](https://github.com/ultralytics/yolov3/commit/444a9f7099d4ff1aef12783704e3df9a8c3aa4b3)
• [BeeDNN - Library in C++](https://github.com/edeforas/BeeDNN)
• [Gen-EfficientNet-PyTorch](https://github.com/rwightman/gen-efficientnet-pytorch)
• [dnet](https://github.com/umangjpatel/dnet/blob/master/nn/activations.py)

All benchmarks, analysis and links to official package implementations can be found in this [repository](https://github.com/digantamisra98/Mish)

It would be nice to have Mish as an option within the activation function group.

The attached image is the comparison of Mish with other conventional activation functions in a SEResNet-50 for CIFAR-10.