Add Issue and PR templates

.github/issue_template.md

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+- [ ] Issue does not already exist
+- [ ] Issue observed on https://plyr.io
+
+### Expected behaviour 
+
+### Actual behaviour
+
+### Environment
+
+- Browser:
+- Version:  
+- Operating System:
+- Version: 
+
+### Steps to reproduce 
+- 
+
+### Relevant links

.github/pull_request_template.md

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+### Link to related issue (if applicable) 
+
+### Sumary of proposed changes 
+
+### Task list
+
+- [ ] Tested on [supported browsers](https://github.com/Selz/plyr#browser-support)
+- [ ] Gulp build completed