add flatMapOkPure function & tests

[adnelson]

Another function that would be really useful for me. LMK if I've overlooked an existing function or if you'd prefer another name

[coveralls]

Pull Request Test Coverage Report for Build 97

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 98.925%

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Totals
Change from base Build 95:	0.03%
Covered Lines:	157
Relevant Lines:	158

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💛 - Coveralls

[briangorman]

@adnelson, LGTM - I think someone has asked for this in the past. Please resolve the conflicts and I will merge

[adnelson]

@briangorman conflicts resolved :)