Flesh out the Composer configuration

Clean up the `config` node, add `composer test` as an alias for PHPUnit, and add script descriptions.

Author: stevegrunwell

composer.json

@@ -20,11 +20,6 @@
     "require-dev": {
         "phpunit/phpunit": ">=6.0"
     },
-    "scripts": {
-        "test-coverage": [
-            "phpunit --coverage-html=tests/coverage"
-        ]
-    },
     "autoload": {
         "psr-4": {
             "SteveGrunwell\\PHPUnit_Markup_Assertions\\": "src/"
@@ -34,5 +29,25 @@
         "psr-4": {
             "Tests\\": "tests/"
         }
+    },
+    "scripts": {
+        "test": [
+            "phpunit"
+        ],
+        "test-coverage": [
+            "phpunit --coverage-html=tests/coverage"
+        ]
+    },
+    "scripts-descriptions": {
+        "test": "Run all test suites.",
+        "test-coverage": "Generate code coverage reports in tests/coverage."
+    },
+    "config": {
+        "preferred-install": "dist",
+        "sort-packages": true,
+        "optimize-autoloader": true,
+        "platform": {
+            "php": "7.2"
+        }
     }
 }