Middleware return early

[anthhub]

I want to add an 'end()' function to provide an option, it's able to backtrack previous middleware for early return.

[codecov]

Codecov Report

Merging #113 (00f1528) into master (25568a3) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 00f1528 differs from pull request most recent head 8157e53. Consider uploading reports for the commit 8157e53 to get more accurate results

@@            Coverage Diff            @@
##            master      #113   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           15        19    +4     
  Branches         5         6    +1     
=========================================
+ Hits            15        19    +4     

Impacted Files	Coverage Δ
index.js	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 25568a3...8157e53. Read the comment docs.

[jonathanong]

why?