feat: add Sequelize to config's declaration and test it works with sequelize-typescript

[Jeff-Tian]

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

• index.d.ts 增加 Sequelize 字段以反映真实的设置，以及支持 TypeScript 里定制这个字段。
• README.md 增加如何在 TypeScript 版 egg 项目里使用 sequelize-typescript 的说明。
• test 目录下增加在 TypeScript 版 egg 项目里使用 sequelize-typescript 的测试用例。
• README.md 增加如何在本地运行此项目的单元测试的说明。
• 增加test.sh 以方便本地单元测试。

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.71%. Comparing base (f585da5) to head (a9f7d6e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #83   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files           4        4           
  Lines          63       63           
=======================================
  Hits           54       54           
  Misses          9        9           

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[Jeff-Tian]

第一个提交测试通过，第二个提交没有本质改动，测试败在 npminstall 这一步。我在 fork 的 TravisCI 也重现了，清除 Cache 后，通过了。

[Jeff-Tian]

@dead-horse @fengmk2 Any chance to get it approved and merged?