stamp_changelog: stamp_date does not support a user passed in date time string

[isadon]

For my changelogs I'm looking to include the time along with the date in the form of 2020-12-08T21:51Z. Can support for passing a string for dates be added?

So for example:

"stamp_date": "2020-12-08T21:51Z", <- takes false, true or a string..

[pajapro]

@donileo just released in https://github.com/pajapro/fastlane-plugin-changelog/releases/tag/v0.16.0