feat: Allow configuration of output YAML file extensions

[heckler1]

Fixes #296

I've retained the current behavior as the default, and added a prop to App to allow configuration of the file extensions applied to the outputted YAML.

Please just let me know if you'd like any more information or if I need to make any changes!

[Chriscbr]

The API looks good to me!

@heckler1 this isn't required, but if you have a chance it would be helpful to document how to use these options on our explanatory docs - I think here could be a good spot for it: https://github.com/cdk8s-team/cdk8s/blob/master/docs/cli/synth.md 😄

[heckler1]

Hey @Chriscbr, happy to add to the docs for sure. I'm a bit unsure where to put this though, I expected that the generated API docs would be synced from this repo into the doc/reference/cdk8s/ folder of the repo you linked, but that doesn't seem to be the case, as those are a few months old now.

I'm not sure that the changes to AppProps really belong in the CLI synth docs, but I can definitely still do a little writeup about Apps and their Props in that file or another one if you'd like.

[Chriscbr]

Ahhh, yeah that's interesting. I did some looking and it looks like the API docs do get updated when we build the website, just not in the repository itself. I'll make a fix to prevent others getting confused by the same thing in the future. 😉

And sure - if you see a better place to put it or want to create a new file about output options, that's fine as well.