[github] Add multiple issue template

[at15]

Previously, GitHub only supports single file template in .github folder, now it supports multiple templates in settings

TODO

• templates for adding/updating database/paper/benchmark
• template about generic issues, like [web] Switch to vuepress from nuxt.js #52 Add license, commercial support info to databases #34

Ref

• https://blog.github.com/2018-05-02-issue-template-improvements/
• https://help.github.com/articles/creating-issue-templates-for-your-repository/