Add issue and pull_request templates. (#713)

* Add issue and pull_request templates.

* Scratch itches

* Respond to review comments.

* Add Stack Overflow link.

* Add impact to issue template, type-of-change to pr.

* Reorder impact, development phase; add development phase to issue.

.github/ISSUE_TEMPLATE.md

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+**Note: for support questions, you are strongly advised to use [Stack Overflow](https://stackoverflow.com/questions/tagged/chisel)**.
+
+This repository's issues are reserved for feature requests and bug reports.
+
+* **Type of issue**
+  - [ ] Bug report
+  - [ ] Feature request
+  - [ ] Other enhancement
+
+* **If the current behavior is a bug, please provide the steps to reproduce the problem:**
+  * **What is the current behavior?**
+  * **What is the expected behavior?**
+  * **Please tell us about your environment:**
+    (examples)
+    - version: `3.0-SNAPSHOT`
+    - OS: `Linux knight 4.4.0-92-generic #115-Ubuntu SMP Thu Aug 10 09:04:33 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux`
+
+* **What is the use case for changing the behavior?**
+
+* **Impact**
+  - [ ] no functional change
+  - [ ] API addition (no impact on existing code)
+  - [ ] API modification
+  - [ ] unknown
+
+* **Development Phase**
+  - [ ] request
+  - [ ] proposal
+
+* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. Stack Overflow, gitter, etc)

.github/PULL_REQUEST_TEMPLATE.md

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+* **Related issue** (if applicable)
+
+* **Type of change**
+  - [ ] Bug report
+  - [ ] Feature request
+  - [ ] Other enhancement
+
+* **Impact**
+  - [ ] no functional change
+  - [ ] API addition (no impact on existing code)
+  - [ ] API modification
+
+* **Development Phase**
+  - [ ] proposal
+  - [ ] implementation
+
+* **Release Notes**
+
+Text from here to the end of the body will be considered for inclusion in the release notes for the version containing this pull request.