github: improve issue templates

[SuperITMan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

We provide only one issue template for every type of issue.
The user cannot select any label and we have to do it ourselves even if it is obvious.

Expected behavior

Providing multiple issue templates as proposed by GitHub. Like this, every issue has a clear template with the right label and the needed information (ie: if a bug, there is no need to provide a "motivation" 😊)

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

Tooling improvement