Add issue template and base demo of gridstack.js.

ISSUE_TEMPLATE.md

+## Subject of the issue
+Describe your issue here.
+
+## Your environment
+* version of gridstack.js
+* which browser and its version
+
+## Steps to reproduce
+Tell us how to reproduce this issue. Please provide a working demo, you can use [this template](https://jsfiddle.net/dweiss/dqyvh6vr/) as a base.
+
+## Expected behaviour
+Tell us what should happen
+
+## Actual behaviour
+Tell us what happens instead