Added Proposal issue template.

.github/ISSUE_TEMPLATE/feature-proposal.md

@@ -0,0 +1,17 @@
+---
+name: Feature Proposal
+about: Suggest an idea for this project
+title: "[Proposal]"
+labels: enhancement
+assignees: ''
+
+---
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Is your proposal implemented in Windows Mail?**
+Wino's priority is to catch up with the feature set of Windows Mail first. 
+
+**Additional context**
+Add any other context or screenshots about the feature request here.