Handle promise return for options / typeahead

[Startouf]

Summary

Allows to respond to typeahead / slack options (see Populate message menus dynamically) via a Promise. PR for https://github.com/slackapi/node-slack-interactive-messages/issues/23

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #24 into master will decrease coverage by 0.58%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   32.51%   31.92%   -0.59%     
==========================================
  Files           3        3              
  Lines         163      166       +3     
==========================================
  Hits           53       53              
- Misses        110      113       +3

Impacted Files	Coverage Δ
src/adapter.js	35.34% <0%> (-0.94%)	:arrow_down:

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[aoberoi]

@Startouf this looks super interesting! i'm sorry nobody got back to you for so long, but i'm taking a look at this right now.

[aoberoi]

I left some comments in #23 about this solution and an idea for something a little more generic.

[aoberoi]

@Startouf thank you for getting started on this effort! I’m closing this PR because it’s superseded by #28