Fix for #221 Allow custom WebClient options

[koh110]

Summary

This pull request check off todo in src/App.ts.

https://github.com/slackapi/bolt/blob/9ac97de32d45002a95e414922fc523f132efeb98/src/App.ts#L136

I want to use options such as proxy for security.
So, I did this todo.

Requirements (place an x in each [ ])

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

[codecov]

Codecov Report

Merging #228 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #228     +/-   ##
=========================================
+ Coverage   61.94%   62.24%   +0.3%     
=========================================
  Files           7        7             
  Lines         494      498      +4     
  Branches      136      138      +2     
=========================================
+ Hits          306      310      +4     
  Misses        163      163             
  Partials       25       25

Impacted Files	Coverage Δ
src/App.ts	71.6% <100%> (+0.71%)	⬆️

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[koh110]

Oops, I overlooked this issue is In Progress.

[seratch]

It looks good to me.