Add default return value in setCustomTransformer callback function

[kungpaogao]

Context

• want to support returning default parsed value from custom transformers
• this PR adds support for returning default parsed values if the custom transformer returns false
• resolves Suggestion: Add default return value in setCustomTransformer callback function #50

Changes

• do not return value if custom transformer returns false

Test Plan

• added unit tests to test actual implementation of custom transformers

[souvikinator]

looks good to me. It should be live in the upcoming version. Thanks for contributing :))

[kungpaogao]

yay! feel free to merge when you're ready :)