IOFactory::identify and Custom Reader/Writer

[oleibman]

Fix #4357.identify returns a type, not a class name, and this result is not always usable for createReader. The docs say that it is usable in that manner. Changing the behavior of identify would be a breaking change, but adding an optional parameter (defaulting to current behavior) allowing it to return a class name rather than a type would not. This PR adds that parameter and updates the documentation to suggest its use for the code in question.

To complete this change, createReader now accepts either a type (current behavior) or class name (new). Although it is not particularly identified as a possible problem by the issue, createWriter is similarly changed for consistency's sake.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary