Support mermaid; modernize codebase (#7)

* Support mermaid; modernize codebase

* Improve traits

* Rename methods

Improve coverage and tests

Fix code style, improve coverage (#6)

* Fix code style, improve coverage

* Tiny change

Fix small things in readme

Small improvements (#2)

* Avoid creative use of names potentially break builder

* Use anon class instead of additional class file in tests

* Fix code coverage filter

* Expose builder properties so custom builders can have custom state/transition classes

* Small doc update

* Remove assumption that repository is traversable

* Just use iterators

Various improvements