Add templates for GitHub issues and pull requests

Also add CONTRIBUTING.md, which was originally a page in Wiki

.github/ISSUE_TEMPLATE.md

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+Make sure these boxes are checked before submitting your issue -- thank you!
+
+- [ ] Check [FAQ](https://github.com/noraesae/perfect-scrollbar/wiki/FAQ)
+- [ ] Search if there's already one reported in Issues
+- [ ] Prepare a JSFiddle reproducing the issue
+  - Perfect Scrollbar JSFiddle: https://jsfiddle.net/DanielApt/xv0rrxv3/
+  - With jQuery: https://jsfiddle.net/DanielApt/gbfLazpx/
+- [ ] Provide a page or source code where the issue can be checked

.github/PULL_REQUEST_TEMPLATE.md

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+Thank you very much for your contribution! Please make sure the followings
+are checked.
+
+- [ ] Read [CONTRIBUTING.md](../CONTRIBUTING.md)
+- [ ] Run `gulp` to make sure it builds and lints successfully
+- [ ] Provide the scenario this PR will address(some JSFiddles will be perfect)
+  - Perfect Scrollbar JSFiddle: https://jsfiddle.net/DanielApt/xv0rrxv3/
+  - With jQuery: https://jsfiddle.net/DanielApt/gbfLazpx/
+- [ ] Refer to concerning issues if exist