Update issue templates

.github/ISSUE_TEMPLATE/proposal.md

@@ -0,0 +1,20 @@
+---
+name: Proposal
+about: Suggest an idea for this project
+title: 'proposal: This is a sample title'
+labels: proposal
+assignees: peaceiris
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.