Possibility to register additional schema types

[brotkrueml]

Sometimes it is necessary to add own schema types (e.g. pending types or from another vocabulary).

• The registration for own types (API) is possible.
• The registration for own view helpers is possible (in addition to model classes).
• Additional WebPage types can be added.
• The documentation is adjusted
• An entry to the CHANGELOG.md is available.

This could then be also the start for an own "schema-health" extension which holds the types and properties for the medical section: https://schema.org/docs/meddocs.html