Make pyyaml an optional dependency

[mauvilsa]

• jsonargparse when installed without any optional dependencies, the default parser mode would use json
• if pyyaml is installed, the default parser mode becomes yaml
• for backward compatibility, the signatures extra would include pyyaml

[petergaultney]

I would really love to see this. You do optional extras for everything else.

We've had serious issues with CLI parsing libraries that have conflicting dependencies in the past, and we're scared to introduce a new one. PyYAML 3.13 is old, which is great, but even better would be no required third party dependencies. And I don't really need YAML parsing (and if I did, i'd happily add an 'extra').