Return middleware chain promise from callback()

[PlasmaPower]

The v2.x version of #847, see there for more details.

[codecov-io]

Codecov Report

Merging #848 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #848   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         425    425           
  Branches      101    101           
=====================================
  Hits          425    425

Impacted Files	Coverage Δ
lib/application.js	100% <100%> (ø)	✅

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[PlasmaPower]

I'm going to rebase this - it'll be very helpful in rewriting a test, so I hope it gets merged soon.

[PlasmaPower]

Rebased! Let me know if anything still needs done before merging. As for testing, my new jest test suite will use this feature in a test. Also, I'm not sure this is complicated enough to need its own test.

[bttmly]

🎉 excellent! @jonathanong -- think we can get this merged soon? Also, I'm going to experiment a bit on some patterns for mounting a Koa app inside a Connect-style app and would be happy to contribute some docs about it once I have an approach that feels right.

[PlasmaPower]

Rebased again. Anything blocking merge?

[bttmly]

Awesome! Thanks for pushing this through @PlasmaPower @jonathanong. If there is interest I'm happy to contribute some docs on how to properly embed Koa w/in existing Connect-style app and how to do an incremental migration.