add ability to read values from environment variables

It makes sense to read some values from environment. In particular this
feature is useful in case of running application in containers in ci/cd

See #108 for more details

tests/ahriman/core/configuration/test_shell_interpolator.py

@@ -0,0 +1,15 @@
+import os
+
+from ahriman.core.configuration.shell_interpolator import ShellInterpolator
+
+
+def test_before_get() -> None:
+    """
+    must correctly extract environment variables
+    """
+    interpolator = ShellInterpolator()
+
+    assert interpolator.before_get({}, "", "", "value", {}) == "value"
+    assert interpolator.before_get({}, "", "", "$value", {}) == "$value"
+    assert interpolator.before_get({}, "", "", "$HOME", {}) == os.environ["HOME"]
+    assert interpolator.before_get({}, "", "", "$$HOME", {}) == "$HOME"