Reducers: add default handler (without init action handling)

[jin60641]

Description

Adds a defaultHandler for reducers apply https://github.com/piotrwitek/typesafe-actions/pull/241 's comments

Related issues:

• Resolved #156

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I have linked all related issues above
• [ ] I have rebased my branch

For bugfixes:

• [ ] I have added at least one unit test to confirm the bug have been fixed
• [ ] I have checked and updated TOC and API Docs when necessary

For new features:

• [x] I have added entry in TOC and API Docs
• [x] I have added a short example in API Docs to demonstrate new usage
• [x] I have added type unit tests with dts-jest
• [x] I have added runtime unit tests with dts-jest

[jin60641]

@piotrwitek I've made changes to not handle the init action by default since it's not a public API. Do you also think this structure is appropriate? Additionally, I believe it would be beneficial to have a test code to check if the defaultReducer is not being called when the init action is dispatched. What do you think?