Add initial issue and pull request templates

.github/ISSUE_TEMPLATE.md

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+Please check if a similar issue already exists or has been closed before before
+opening your issue.
+
+### Description
+
+[Description of the bug or feature]
+
+### Expected behavior
+
+[What you expected to happen]
+
+### Actual behavior
+
+[What actually happened]
+
+### Steps to Reproduce
+
+1. [First Step]
+2. [Second Step]
+3. [and so on...]
+
+### Versions
+
+  - Prezto commit:
+  - ZSH version:
+  - OS information:

.github/PULL_REQUEST_TEMPLATE.md

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+Please be sure to check out our [contributing
+guidelines](https://github.com/sorin-ionescu/prezto/blob/master/CONTRIBUTING.md)
+before submitting your pull request.
+
+Fixes #
+
+## Proposed Changes
+
+  -
+  -
+  -