Validate untyped maps similar to structs (#128)

* Validate untyped maps similar to structs

* Map validation refinements based on feedback

* Flag map keys as optional

v4.2.2 release

v4.2.1 release

v4.2.0 release

added documentation for conditional validation

more docs

Fixes #72: support less restrictive equality comparison by the In rule

gofmt fixes

added coverage info

Fixes #64: added example of parameterizing custom rules