refactor: use async function and support egg@2

[dead-horse]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

[codecov-io]

Codecov Report

Merging #10 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #10   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines          58     58           
=====================================
  Hits           58     58

Impacted Files	Coverage Δ
app/extend/application.js	100% <100%> (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 705ffcb...fc364ae. Read the comment docs.

[dead-horse]

@eggjs/core