Added option for custom packager port

[Gongreg]

Issue: #1367

What I did

Added option for custom packager port (--packager-port=xxxx) .

How to test

Pass option --packager-port and see that new port is used.

Is this testable with jest or storyshots?
RN cli is not tested with jest yet.

Does this need a new example in the kitchen sink apps?
No.

Does this need an update to the documentation?
Not sure. We should document all the options available in cli.

If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #1837 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1837      +/-   ##
==========================================
+ Coverage   21.32%   21.33%   +<.01%     
==========================================
  Files         257      257              
  Lines        5739     5737       -2     
  Branches      686      686              
==========================================
  Hits         1224     1224              
+ Misses       3981     3979       -2     
  Partials      534      534

Impacted Files	Coverage Δ
app/react-native/src/bin/storybook-start.js	0% <0%> (ø)	⬆️

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[ndelangen]

If you have tested this @Gongreg, go ahead and merge.