Add labels to issue templates

Make use of https://blog.github.com/changelog/2018-12-05-issue-template-automation-improvements/

Author: dwijnand

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,6 +1,7 @@
 ---
 name: Bug report
 about: Create a report to help us improve
+labels: C-bug
 ---
 
 <!-- Thanks for filing a 🐛 bug report 😄! -->

.github/ISSUE_TEMPLATE/feature_request.md

@@ -1,6 +1,7 @@
 ---
 name: Feature request
 about: Suggest an idea for this project
+labels: C-feature-request
 ---
 
 <!-- Thanks for filing a 🙋 feature request 😄! -->