Update issue templates (#1636)

This updates issue templates to support auto-labeling, and multiple issue formats.

.github/ISSUE_TEMPLATE/announcement.md

@@ -0,0 +1,10 @@
+---
+name: Announcement
+about: Announcements related to the project
+title: "[Announcement] TITLE"
+labels: Announcement
+assignees: ''
+
+---
+
+

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,54 @@
+---
+name: Bug report
+about: Create a bug report to help us improve
+title: "[BUG] Description"
+labels: 'Issue: Bug'
+assignees: ''
+
+---
+
+<!--
+Please follow the issue template below for bug reports.
+Also please indicate in the issue title which language/library is concerned. Eg:  [BUG][JAVA] Bug generating foo with bar 
+-->
+
+##### Description
+
+<!-- describe what is the question, suggestion or issue and why this is a problem for you. -->
+
+##### openapi-generator version
+
+<!-- which version of openapi-generator are you using, is it a regression? -->
+
+##### OpenAPI declaration file content or url
+
+<!-- if it is a bug, a json or yaml that produces it.
+If you post the code inline, please wrap it with
+```yaml
+(here your code)
+```
+(for YAML code) or
+```json
+(here your code)
+```
+(for JSON code), so it becomes more readable. If it is longer than about ten lines,
+please create a Gist (https://gist.github.com) or upload it somewhere else and
+link it here.
+  -->
+
+##### Command line used for generation
+
+<!-- including the language, libraries and various options -->
+
+##### Steps to reproduce
+
+<!-- unambiguous set of steps to reproduce the bug.-->
+
+##### Related issues/PRs
+
+<!-- has a similar issue/PR been reported/opened before? Please do a search in https://github.com/openapitools/openapi-generator/issues?utf8=%E2%9C%93&q=is%3Aissue%20 -->
+
+##### Suggest a fix
+
+<!-- if you can't fix the bug yourself, perhaps you can point to what might be
+  causing the problem (line of code or commit), or simply make a suggestion -->

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,24 @@
+---
+name: Feature request
+about: Suggest an idea for this project
+title: "[REQ] Feature Request Description"
+labels: 'Enhancement: Feature'
+assignees: ''
+
+---
+
+### Is your feature request related to a problem? Please describe.
+
+<!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
+
+## Describe the solution you'd like
+
+<!-- A clear and concise description of what you want to happen. -->
+
+## Describe alternatives you've considered
+
+<!-- A clear and concise description of any alternative solutions or features you've considered. -->
+
+## Additional context
+
+<!-- Add any other context or screenshots about the feature request here. -->