Add support connect style middleware

[hoangvvo]

With the release of my next-connect, it is now possible to use Connect-style middleware (function (req, res, next) {})

This PR attempts to have the deprecated default export to have this.

[codecov]

Codecov Report

Merging #39 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   94.11%   94.08%   -0.04%     
==========================================
  Files           6        6              
  Lines         187      186       -1     
==========================================
- Hits          176      175       -1     
  Misses         11       11

Impacted Files	Coverage Δ
src/index.js	92.07% <100%> (-0.08%)	⬇️

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[hoangvvo]

Code coverage will be hit. The PR to fix #38 will hopefully fix this.