fix(NodeAttrs): lazy init of observer listeners
style(events): code cleanup
chore(CONTRIBUTING): explain how to squash and copyedit

- Explain how to squash multiple commits.
- Copyedit.

Closes #740
fix(tagging compiler): Empty transclusions
fix(tagging compiler): Transclusions with an existing ElementBinder
chore(dom): toString for debugging
fix(tagging compiler): Sibling templates
refactor(forms): use errorStates and warningStates

Closes #736
Closes #706
Fixes #724
chore(tests): remove inject() calls
feat(hg-model): support input type=date|datetime|...

Add support for input elements of type
date|datetime|datetime-local|month|time|week.