Allow output to be configurable

Looking at things like the header section I can see how it would be useful once it's clear what each heading means (It took me a few minutes to work out how "Abstract" and "Extensible" are defined for example).

It could be useful for those looking for simple output, to allow things like the header to be configurabally turned off.

I can see how a number of other elements may want to be configurable, so I'd suggest something like a jsonschema2md.rc file.

```
{
"config": {
"sections": {
"header": false
}
},
"meta": {
"authors": [
{
"name": "Mr Test"
}
]
}
}
```

or for alternate header layout, assuming the current layout is "table"...

```
{
"config": {
"sections": {
"header": {
"layout": "list"
}
}
}
}
```

Does this sound reasonable?

If so, I'll begin work on making this happen.

Start by tracing how the current header section is generated and how command input is configured. Decide the supported configuration shape for disabling sections and selecting an alternate header layout, then verify the documented examples produce the intended Markdown output. The issue does not name implementation files or tests, so these entry points must be identified first.

Written by the indexing model from the issue text.