Add fx.ShutdownError option

[abramlab]

Adds the fx.ShutdownError option, to be able to pass a custom error to Shutdowner and retrieve it from the application as the reason it shutdown.

Refers #672

[CLAassistant]

All committers have signed the CLA.

[jasonmills]

@abramlab can you rebase this PR to make sure there aren't any conflicts against the latest changes to the shutdown and signal logic: https://github.com/uber-go/fx/commit/b379e132182b47a3f2400212e136898575665154

[abramlab]

@abramlab can you rebase this PR to make sure there aren't any conflicts against the latest changes to the shutdown and signal logic: b379e13

Rebase done

[codecov[bot]]

Codecov Report

Merging #996 (bbd8c2a) into master (ed3eef5) will decrease coverage by 0.05%. The diff coverage is 100.00%.

:exclamation: Current head bbd8c2a differs from pull request most recent head a285dde. Consider uploading reports for the commit a285dde to get more accurate results

@@            Coverage Diff             @@
##           master     #996      +/-   ##
==========================================
- Coverage   98.09%   98.04%   -0.05%     
==========================================
  Files          39       39              
  Lines        1995     1996       +1     
==========================================
  Hits         1957     1957              
- Misses         30       31       +1     
  Partials        8        8              

Impacted Files	Coverage Δ
shutdown.go	100.00% <100.00%> (ø)
annotated.go	99.00% <0.00%> (-0.17%)	:arrow_down:

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[abramlab]

@jasonmills what else is needed to merge the pull request?