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

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (41c4358) to head (8266514).
Report is 8 commits behind head on master.

Additional details and impacted files

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

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