Add issue templates (#265)

We get a _lot_ of issues that match the pattern of `FAILING_CONNECTION`
or `FEAUTURE_REQUEST` which can't be solved in this repository. Document
the common stuff to hopefully get fewer errant issues filed.

Author: natebosch

.github/ISSUE_TEMPLATE/BUG_REPORT.md

@@ -0,0 +1,17 @@
+---
+name: I found a bug.
+about: You found that something which is expected to work, doesn't. The same
+  capability works when using `dart:io` or `dart:html` directly.
+
+---
+
+<!--
+  Please describe the bug and how to reproduce it.
+
+  Note that if the bug can also be reproduced when going through the interfaces
+  provided by `dart:html or `dart:io` directly the bug should be filed against
+  the Dart SDK: https://github.com/dart-lang/sdk/issues
+
+  A failure to make an http request is more often a problem with the environment
+  than with the client.
+-->

.github/ISSUE_TEMPLATE/FAILING_CONNECTION.md

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+---
+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 clients [`dart:io` client][] and
+[`dart:hmtl` request][] to give a unified interface. Before filing a bug here,
+verify that the issue is not surfaced when using those interfaces directly.
+
+[`dart:io` client]: https://api.dartlang.org/stable/dart-io/HttpClient-class.html
+[`dart:html` request]: 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][]
+
+[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.

.github/ISSUE_TEMPLATE/FEATURE_REQUEST.md

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+---
+name: I'd like a new feature.
+about: You are using `package:http` and would like a new feature to make it
+  easier to make http requests.
+
+---
+
+<!--
+  Please describe the feature you'd like to see us implement along with a use
+  case.
+
+
+  Note that this package is designed to be cross-platform and we will only be
+  able to add features which can be supported with _both_ `dart:io` and
+  `dart:html`. If you're looking for a feature which is already supported by the
+  `daart:io` HttpClient constructor you can construct one manually and pass it
+  to the IOClient constructor directly.
+-->