Accept all `pyyam`l types as valid types.

[Rizhiy]

I need to use dates in my config.
When I put in my date in the yaml file:

DATE: 2020-01-01

it is automatically converted to datetime.date by yaml.safe_load.
This then breaks the loading since date is not a valid type.

Currently, I have to explicitly state that the value is a string using !!str, this is cumbersome.

I suggest that yacs should support all types supported by pyyaml.

[Rizhiy]

Forgot I already opened similar issue, before will move this there.