Added pull request and issue templates

The issue template is largely based on the questions included in the
canned comment that I use for bugs, which is mostly covered in the
contributing guide. Even though there are a lot of lines, most of it is
just ticking checkboxes.

The issue template is focused on bug reports, as those are considerably
more common than feature requests.

The pull request template was designed to be pretty slim, we haven't had
many issues with these in the past.

ISSUE_TEMPLATE.md

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+## Prerequisites
+
+- [ ] I have searched for similar issues in both open and closed tickets and cannot find a duplicate
+- [ ] The issue still exists against the latest `master` branch of Select2
+- [ ] This is not a usage question (Those should be directed to the [community](https://select2.github.io/community.html))
+- [ ] I have attempted to find the simplest possible steos to reproduce the issue
+- [ ] I have included a failing test as a pull request (Optional)
+
+## Steps to reproduce the issue
+
+1.
+2.
+3.
+
+## Expected behavior and actual behavior
+
+When I follow those steps, I see...
+
+I was expecting...
+
+## Environment
+
+Browsers
+
+- [ ] Google Chrome
+- [ ] Mozilla Firefox
+- [ ] Internet Explorer
+
+Operating System
+
+- [ ] Windows
+- [ ] Mac OS X
+- [ ] Linux
+- [ ] Mobile
+
+Libraries
+
+- jQuery version:
+- Select2 version:
+
+## Isolating the problem
+
+- [ ] This bug happens [on the examples page](https://select2.github.io/examples.html)
+- [ ] The bug happens consistently across all tested browsers
+- [ ] This bug happens when using Select2 without other pluigns
+- [ ] I can reproduce this bug in [a jsbin](https://jsbin.com/)

PULL_REQUEST_TEMPLATE.md

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+This pull request includes a
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Translation
+
+The following changes were made
+
+-
+-
+-
+
+If this is related to an existing ticket, include a link to it as well.