Add issue & PR templates to this repository

[cmaumet]

To ease the process of adding new contributions to the repository, we could create templates for issues and pull requests: cf. https://help.github.com/articles/creating-an-issue-template-for-your-repository/.

[ABL4Z3]

Hi, I am a new contributor interested in helping with this.
I would like to add basic issue templates (bug report and feature request) and a pull request template following the GitHub docs you linked.