feat: support modelDir option

[xiaoxiaocoder]

Checklist

• npm test passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Description of change

• add modelDir option to support custom model directory(default: model)
• add delegate option to avoid confilct with egg-sequelize's delegate

[codecov]

Codecov Report

Merging #42 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #42   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           63        64    +1     
=========================================
+ Hits            63        64    +1     

Impacted Files	Coverage Δ
config/config.default.js	100.00% <ø> (ø)
lib/mongoose.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 41c4358...8266514. Read the comment docs.