Add config.parse_bool() function

For those generic needs...

rattail/config.py

@@ -43,6 +43,19 @@ from rattail.logging import TimeConverter
 log = logging.getLogger(__name__)
 
 
+def parse_bool(value):
+    """
+    Derive a boolean from the given string value.
+    """
+    if value is None:
+        return None
+    if isinstance(value, bool):
+        return value
+    if value.lower() in ('true', 'yes', 'on', '1'):
+        return True
+    return False
+
+
 def parse_list(value):
     """
     Parse a configuration value, splitting by whitespace and/or commas and
@@ -245,13 +258,7 @@ class RattailConfig(object):
         Retrieve a boolean value from config.
         """
         value = self.get(*args, **kwargs)
-        if value is None:
+        return parse_bool(value)
-            return None
-        if isinstance(value, bool):
-            return value
-        if value.lower() in ('true', 'yes', 'on', '1'):
-            return True
-        return False
 
     def getint(self, *args, **kwargs):
         """