[skip travis] Added enhancement issue template

Author: s1hofmann

.github/ISSUE_TEMPLATE/enhancement.md

@@ -0,0 +1,13 @@
+---
+name: Enhancement
+about: Suggest a possible enhancement to the project
+---
+
+**Short overview**
+
+**Use case**
+
+**Detailed description**
+
+**Additional content**
+> Please provide any (mandatory) additional data for your enhancement