Add jvm parser configuration

[e13mort]

Hello. I use your library in one of my projects and have faced with an interesting issue.

In my case I have files with DTD declaration with wrong Url. I don't manage these files so can't remove that declaration manually. So, that incorrect declaration leads to normal work at jvm(android), native and js targets. But, plain jvm (javax.xml) fails with a valid reason The markup declarations contained or pointed to by the document type declaration must be well-formed. In that particular case I'm allowed to ignore that check.

The problem there:

nl.adaptivity.xmlutil.StAXReader.Companion#safeInputFactory has the following code:

return XMLInputFactory.newFactory().apply {
          setProperty(XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES, false)
     }

I can't pass any additional properties to that factory. In my case I need to set property javax.xml.stream.supportDTD to false (and it works via debugger).

I see few ways how it might be implemented:

• create a high-level property somewhere in the nl.adaptivity.xmlutil.serialization.XmlConfig.Builder (I guess). It's gonna blow API a little but will be suitable for my case and kind of easy to implement.
• Simply disable DTD check in the code above. It's gonna be a downgrade to jvm target but will make the whole library more consistent (as far as it's already not working on other platforms).
• Add a feature to fully configure platform implementations. Looks like the right way in general but will require more work.

What do you think?