New issue templates

[link3280]

Currently, we have mainly two types of issues: bug reports and feature requests, and some efforts (e.g. refactoring for code style/performance) may not fall into these categories. So I propose to add an improvements template for these efforts.

[yaooqinn]

I am OK with this cc @yanghua

[yanghua]

+1, sorry for the late reply, I opened "All Activity" in the Github "Watch". So I was inundated by so many messages.