Add support for a global configuration file.

Closes #63

[email protected]

Review URL: https://codereview.chromium.org//1829483002 .

Author: nex3

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 0.12.13
 
+* Add support for a global configuration file. On Windows, this file defaults to
+  `%LOCALAPPDATA%\DartTest.yaml`. On Unix, it defaults to `~/.dart_test.yaml`.
+  It can also be explicitly set using the `DART_TEST_CONFIG` environment
+  variable. See [the configuration documentation][global config] for details.
+
 * The `--name` and `--plain-name` arguments may be passed more than once, and
   may be passed together. A test must match all name constraints in order to be
   run.
@@ -13,6 +18,8 @@
 * Add a `pause_after_load` field to the package configuration file. This allows
   presets to enable debugging mode.
 
+[global config]: https://github.com/dart-lang/test/blob/master/doc/configuration.md#global-configuration
+
 ## 0.12.12
 
 * Add support for [test presets][]. These are defined using the `presets` field