Update GitHub issue and PR templates (#609)

The previous content was generically copied from an ancient AspNet
template, and had examples that were wholly unrelated to LibMan.  Having
a better template will make it more clear how to file good issues.

Addresses #129.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,24 @@
+---
+name: Bug report
+about: Create a report about something that is not working
+---
+
+### Describe the bug
+A clear and concise description of what the bug is.
+
+### To Reproduce
+Steps to reproduce the behavior:
+1. Using this version of Visual Studio '...'
+2. Take this action...
+4. See error
+
+### Expected behavior
+A clear and concise description of what you expected to happen.
+
+### Screenshots
+If applicable, add screenshots to help explain your problem.
+
+### Additional context
+Add any other context about the problem here.
+If using the LibMan CLI tool, include the output of `libman --version`
+If using Visual Studio, include the version of Visual Studio and of `Microsoft Library Manager` from the Installed Products section of the About dialog.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,17 @@
+---
+name: Feature request
+about: Suggest an idea for this project
+---
+
+### Is your feature request related to a problem? Please describe.
+A clear and concise description of what the problem is.
+Example. I'm am trying to do [...] but [...]
+
+### 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.

.github/pull_request_template.md

@@ -0,0 +1,5 @@
+Summary of the changes (Less than 80 chars)
+ - Detail 1
+ - Detail 2
+
+Addresses #bugnumber (in this specific format)