add config validator subcommand (#80)

* add config validator subcommand

* add --exit-code flag

* docs & faq update

tests/ahriman/core/formatters/conftest.py

@@ -1,7 +1,7 @@
 import pytest
 
 from ahriman.core.formatters import AurPrinter, ConfigurationPrinter, PackagePrinter, PatchPrinter, StatusPrinter, \
-    StringPrinter, TreePrinter, UpdatePrinter, UserPrinter, VersionPrinter
+    StringPrinter, TreePrinter, UpdatePrinter, UserPrinter, ValidationPrinter, VersionPrinter
 from ahriman.models.aur_package import AURPackage
 from ahriman.models.build_status import BuildStatus
 from ahriman.models.package import Package
@@ -126,6 +126,29 @@ def user_printer(user: User) -> UserPrinter:
     return UserPrinter(user)
 
 
+@pytest.fixture
+def validation_printer() -> ValidationPrinter:
+    """
+    fixture for validation printer
+
+    Returns:
+        ValidationPrinter: validation printer test instance
+    """
+    return ValidationPrinter("root", [
+        "root error",
+        {
+            "child": [
+                "child error",
+                {
+                    "grandchild": [
+                        "grandchild error",
+                    ],
+                },
+            ],
+        },
+    ])
+
+
 @pytest.fixture
 def version_printer(package_ahriman: Package) -> VersionPrinter:
     """