add feature request template (#219)

Author: okue

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,17 @@
+---
+name: Feature Request
+about: Suggest an idea for this project
+labels: enhancement
+
+---
+
+## Do this before creating an issue
+<!-- Delete this section before the submit -->
+
+- Check our existing issues, pull requests, and [SDK Document](https://line-bot-sdk-python.readthedocs.io/en/latest/).
+
+## Problem
+<!-- Describe a clear and concise description of what the problem is. -->
+
+## Solution
+<!-- Describe a clear and concise description of what you want to happen. -->