add issue and PR templates

ISSUE_TEMPLATE.md

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+Please check the existing issues to make sure you're not duplicating a report.
+
+For bug reports, please provide the following information:
+
+- Zim commit ref:
+- gist of .zimrc:
+- Zsh version:
+- gist of .zshrc:
+- $(uname -mosr):
+
+Description
+-----------
+
+
+Steps to reproduce
+------------------
+
+
+Images or other information
+---------------------------
+
+
+

PULL_REQUEST_TEMPLATE.md

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+Please follow Zim's [code style guidelines](https://github.com/Eriner/zim/wiki/Code-Style-Guide).