Fix callback handling, add options

[aoberoi]

Summary

fixes #23 fixes #25

adds two new configuration options for SlackMessageAdapter:

• syncResponseTimeout - number of milliseconds to wait before flushing a syncrhonous response to an incoming request and falling back to an asynchronous response.
• lateResponseFallbackEnabled - whether or not promises that resolve after the syncResponseTimeout can fallback to a request for the response_url. this only works in cases where the semantic meaning of the response and the response_url are the same.

this impacts the table in #27 by making it look like the following:

Type of Request	cb(): Message	cb(): Promise<Message>	respond(m: Message)	respond(m: Promise<Message>)	Notes
Message Action (Button, Select) incl. embeded in App Unfurl	synchronous response	if resolved under timeout, synchronous response; else response_url request	response_url request (can be used several times)	error: axios will likely throw
Dialog Submission	synchronous response	synchronous response	response_url request (can be used several times) - not a replacement, used as follow up	error: axios will likely throw	A warning is logged if the synchronous response takes longer than the timeout
Menu Options	synchronous response	synchronous response	-	-	A warning is logged if the synchronous response takes longer than the timeout

TODO:

• [x] tests
• [x] creating config variable for timeout
• [x] better error message when calling respond() with a Promise (maybe)
• [x] update deps?

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.

[codecov[bot]]

Codecov Report

Merging #28 into master will increase coverage by 46.55%. The diff coverage is 92.3%.

@@             Coverage Diff             @@
##           master      #28       +/-   ##
===========================================
+ Coverage   32.51%   79.06%   +46.55%     
===========================================
  Files           3        3               
  Lines         163      172        +9     
===========================================
+ Hits           53      136       +83     
+ Misses        110       36       -74

Impacted Files	Coverage Δ
src/express-middleware.js	14.63% <0%> (-0.76%)	:arrow_down:
src/util.js	100% <100%> (+45.45%)	:arrow_up:
src/adapter.js	99.14% <100%> (+62.86%)	:arrow_up:

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[aoberoi]

in the interest of building new features on top of this branch, and since there aren't yet any reviewers with specific expertise in this module, i'm going to merge. general JS code review still welcome!