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

Merging #83 (8a3c1f4) into master (f585da5) will decrease coverage by 1.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   85.71%   84.37%   -1.34%     
==========================================
  Files           4        4              
  Lines          63       64       +1     
==========================================
  Hits           54       54              
- Misses          9       10       +1     

Impacted Files	Coverage Δ
lib/loader.js	83.05% <100.00%> (-1.44%)	⬇️

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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?