Add support for .yaml extension lookup of boilerplate file.

[Rakhmanov]

Describe the solution you'd like
Add support to lookup boilerplate.yaml in addition to boilerplate.yml.

Describe alternatives you've considered
This is first project I've seen that is strict about extension choice, intentionally or not. This is deviates from expectation and requires additional automated verification or mental toil to avoid committing files with unsupported extension. It will be much easier if the both idiomatic exceptions would be supported.

Additional context
Currently there valid boilerplate file with extension .yaml is not being found. It would be good practice not to prescribe usage of specific extensions and leave it to the user's team and repository conventions to drive this decision.