Add template for issues and pull requests (#1116)

.github/ISSUE_TEMPLATE.md

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+Too many issues will kill our team's development velocity, drastically.
+Make sure you have checked all steps below.
+
+### Prerequisite
+* [ ] Make sure your problem is not listed in [the common build problems](https://github.com/pyenv/pyenv/wiki/Common-build-problems)
+* [ ] Make sure duplicated issue has already been reported in [the pyenv issues](https://github.com/pyenv/pyenv/issues). You should look for closed issues too.
+* [ ] Make sure you are not asking us to help solving your specific issue.
+  * GitHub issues is opened mainly for development purpose. If you want to ask someone to help solving your problem, go some community site like [Gitter](https://gitter.im/yyuu/pyenv), [StackOverflow](https://stackoverflow.com/questions/tagged/pyenv), etc.
+* [ ] Make sure your problem is not derived from packaging (e.g. [Homebrew](https://brew.sh))
+  * Please refer the package documentation for the installation issues, etc.
+* [ ] Make sure your problem is not derived from plugins
+  * This repository is maintaining `pyenv` and the default `python-build` plugin only. Please refrain to report the issues of the plugin here.
+
+### Description
+- [ ] Platform information (e.g. Ubuntu Linux 16.04):
+- [ ] OS architecture (e.g. amd64):
+- [ ] pyenv version:
+- [ ] Python version:
+- [ ] C Compiler information (e.g. gcc 7.3): 
+- [ ] Please attach verbose build log as gist
+  * You can turn on verbose debug log like `env PYENV_DEBUG=1 pyenv install -v 3.6.4`

.github/PULL_REQUEST_TEMPLATE.md

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+Make sure you have checked all steps below.
+
+### Prerequisite
+* [ ] Please consider implementing the feature as hook script or plugin at first
+  * pyenv has some powerful support for plugins and hook scripts. Please refer [Authoring plugins](https://github.com/pyenv/pyenv/wiki/Authoring-plugins) for details and try to implement as some plugin if possible.
+* [ ] Please consider contribute similar patch to [rbenv](https://github.com/rbenv/rbenv) since we have borrowed most of lines with the project
+  * We are occasionally importing the changes from the rbenv. In general, you can expect some changes made in rbenv will be imported to pyenv too, eventually.
+  * Generaly speaking, we sometime don't prefer to make some change in the core feature to keep compatibility with rbenv.
+* [ ] My PR addresses the following pyenv issue (if any)
+  - https://github.com/pyenv/pyenv/issues/XXXX
+
+### Description
+- [ ] Here are some details about my PR
+
+### Tests
+- [ ] My PR adds the following unit tests (if any)