Empyrial

- Fixed optimizers and more options to [customize expected returns and risk model](https://github.com/ssantoshp/Empyrial/blob/3207c0de8d109db70bee8f9c8dbfa881cdeeb967/src/empyrial/main.py#L103)
- Support for custom data! `data` field has been added to the Engine class. Check the doc here to learn how to use it: https://empyrial.gitbook.io/empyrial/
- Some code refactoring, `get_report` has been deleted. To generate a report, use `empyrial(portfolio, report=True)`.

Quick fix

Fixed TypeError that occurs when a default value (current date) is passed to the Engine.

77 and 104 fixed, credit goes to pzzhang