feat: add withObjectMapper method to customize serialization of schemas to file

[takanuva15]

Hi, I put together this rough PR based on our discussion in #384.

3 quick questions on this:

1. Assuming the logic looks fine to you, can you clarify which test classes I need to update? (So far I found SchemaGeneratorConfigBuilderTest.java).
2. Do I need to update the github.io documentation you have published on the gh-pages branch?
3. Is it worthwhile to add an example within jsonschema-examples demonstrating the customization of the ObjectMapper to print yaml?

[CarstenWickner]

Thanks for raising this PR 🙇

To answer your questions:

1. The general logic fine, yes. I just added two minor comments. I have no particular test classes in mind, but yes the SchemaGeneratorConfigBuilderTest would be good for completeness' sake. The change is not that big I suppose.
2. I can take care of the documentation afterward. For reference though, it's defined as markdown files in the slate-docs/source/includes directory of this repository.
3. Examples for the Maven plugin are somewhat tricky in the jsonschema-examples scope. An example of using YAML as the output format would be good to include in the FAQs of the documentation I suppose. Feel free to add it yourself, otherwise, I'll do it later.

[takanuva15]

To answer your questions:
...

Thanks for the reply. I've updated the PR and left the documentation untouched.

This change is really only relevant to the maven plugin, because if a dev is manually making their own schema through the builder, then they'll have access to the root JsonNode and can serialize it however they want.

In that sense, perhaps the documentation would make more sense to go under _maven-plugin.md. In any case, I'll leave it to you to configure it as you like.

[CarstenWickner]

Looking good now.
Adding this to the documentation of the Maven plugin is a fair point.
Users are more likely to find it there than in the FAQs.

I'll do that in a separate PR then.
Thank you for this contribution. 😄👍