add support for adding reference links to values

Now Value classes accept a 'help_reference' keyword which will get printed when a
ValueRetrievalError or ValueProcessingError occurs

configurations/errors.py

@@ -19,6 +19,9 @@ def extract_explanation_lines_from_value(value_instance: 'Value') -> List[str]:
     if value_instance.help_text is not None:
         result.append(f"Help: {value_instance.help_text}")
 
+    if value_instance.help_reference is not None:
+        result.append(f"Reference: {value_instance.help_reference}")
+
     if value_instance.destination_name is not None:
         result.append(f"{value_instance.destination_name} is taken from the environment variable "
                       f"{value_instance.full_environ_name} as a {type(value_instance).__name__}")

configurations/values.py

@@ -59,7 +59,7 @@ class Value:
 
     def __init__(self, default=None, environ=True, environ_name=None,
                  environ_prefix='DJANGO', environ_required=False, help_text=None,
-                 *args, **kwargs):
+                 help_reference=None, *args, **kwargs):
         if isinstance(default, Value) and default.default is not None:
             self.default = copy.copy(default.default)
         else:
@@ -72,6 +72,7 @@ class Value:
         self.environ_required = environ_required
         self.destination_name = None
         self.help_text = help_text
+        self.help_reference = help_reference
 
     def __str__(self):
         return str(self.value)

test_project/test_project/settings.py

@@ -4,7 +4,8 @@ from configurations import Configuration, values
 class Base(Configuration):
     # Django settings for test_project project.
 
-    DEBUG = values.BooleanValue(True, environ=True, help_text="Enables or disables django debug mode")
+    DEBUG = values.BooleanValue(True, environ=True, help_text="Enables or disables django debug mode",
+                                help_reference="https://docs.djangoproject.com/en/dev/ref/settings/#debug")
 
     ADMINS = (
         # ('Your Name', 'your_email@example.com'),