Tweak formatting of template (#266)

The default view the user will see is not markdown rendered - it's the
text directly. Update formatting to look a bit nicer in that view since
we don't care what the rendered view will look like - we don't expect
the issue to get submitted.

Also prefix numbers to the filenames to order the issues.

Author: natebosch

.github/ISSUE_TEMPLATE/1-FAILING_CONNECTION.md

@@ -0,0 +1,35 @@
+---
+name: An HTTP request fails when made through this client.
+about: You are attempting to make a GET or POST and getting an unexpected
+  exception or status code.
+
+---
+
+**********************************************************
+**This is not the repository you want to file issues in!**
+**********************************************************
+
+Note that a failing HTTP connection is almost certainly not caused by a bug in
+package:http.
+
+This package is a wrapper around the HttpClient from dart:io and HttpRequest
+from dart:html. Before filing a bug here verify that the issue is not surfaced
+when using those interfaces directly.
+
+https://api.dartlang.org/stable/dart-io/HttpClient-class.html
+https://api.dartlang.org/stable/dart-html/HttpRequest-class.html
+
+# Common problems:
+
+- A security policy prevents the connection.
+- Running in an emulator that does not have outside internet access.
+- Using Android and not requesting internet access in the manifest.
+  https://github.com/flutter/flutter/issues/29688
+
+
+None of these problems are influenced by the code in this repo.
+
+# Diagnosing:
+
+- Attempt the request outside of Dart, for instance in a browser or with `curl`.
+- Attempt the request with the dart:io or dart:html equivalent code paths.