allow yes/no as boolean values

[wookietreiber]

works:

foo: [true|false]

doesn't work:

foo: [yes|no]

... invalid type: string "yes", expected a boolean at line ...
... invalid type: string "no", expected a boolean at line ...

are you open to support yes and no for boolean values? I'd be open to provide a PR if it's not too difficult to implement.

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bit of background: I've been doing lots of work with Ansible, which allows yes and no. with Ansible, I'm even encouraging it by disallowing true and false with yamllint because it usually feels/reads more natural when reading/writing Ansible code.

[dtolnay]

This is a duplicate of #379.