feature: Custom runner support for time series models like ARIMA using BentoML

[jsamanta]

Feature request

I have implemented and tested a feature that trains a time series model - ARIMA and serves it using BentoML's custom runner to make predictions.

Motivation

I believe it would be a nice guide to someone exploring time series models with custom runner implementation for models like ARIMA.

Other

I would like to submit a PR but it would be nice if you could point me to as where I can push this code so that its available to users.