[procedure] Added support for custom values on the return statement.

This enables constructs like this in procedures:

```yaml
- return

- return 1

- return: ${output}
```

platypush/procedure/__init__.py

@@ -1,10 +1,11 @@
 import enum
 import logging
 import re
+from dataclasses import dataclass
 from functools import wraps
 
 from queue import LifoQueue
-from typing import Optional
+from typing import Any, Optional
 
 from ..common import exec_wrapper
 from ..config import Config
@@ -14,7 +15,7 @@ from ..message.response import Response
 logger = logging.getLogger('platypush')
 
 
-class Statement(enum.Enum):
+class StatementType(enum.Enum):
     """
     Enumerates the possible statements in a procedure.
     """
@@ -24,6 +25,45 @@ class Statement(enum.Enum):
     RETURN = 'return'
 
 
+@dataclass
+class Statement:
+    """
+    Models a statement in a procedure.
+    """
+
+    type: StatementType
+    argument: Optional[str] = None
+
+    @classmethod
+    def build(cls, statement: str):
+        """
+        Builds a statement from a string.
+        """
+
+        m = re.match(r'\s*return\s*(.*)\s*', statement, re.IGNORECASE)
+        if m:
+            return ReturnStatement(argument=m.group(1))
+
+        return cls(StatementType(statement.lower()))
+
+    def run(self, *_, **__) -> Optional[Any]:
+        """
+        Executes the statement.
+        """
+
+
+@dataclass
+class ReturnStatement(Statement):
+    """
+    Models a return statement in a procedure.
+    """
+
+    type: StatementType = StatementType.RETURN
+
+    def run(self, *_, **context):
+        return Request.expand_value_from_context(self.argument, **context)
+
+
 class Procedure:
     """Procedure class. A procedure is a pre-configured list of requests"""
 
@@ -70,7 +110,15 @@ class Procedure:
         for request_config in requests:
             # Check if it's a break/continue/return statement
             if isinstance(request_config, str):
-                reqs.append(Statement(request_config))
+                reqs.append(Statement.build(request_config))
+                continue
+
+            # Check if it's a return statement with a value
+            if (
+                len(request_config.keys()) == 1
+                and list(request_config.keys())[0] == 'return'
+            ):
+                reqs.append(ReturnStatement(argument=request_config['return']))
                 continue
 
             # Check if this request is an if-else
@@ -218,15 +266,16 @@ class Procedure:
                 continue
 
             if isinstance(request, Statement):
-                if request == Statement.RETURN:
+                if isinstance(request, ReturnStatement):
+                    response = Response(output=request.run(**context))
                     self._should_return = True
                     for proc in __stack__:
                         proc._should_return = True  # pylint: disable=protected-access
                     break
 
-                if request in [Statement.BREAK, Statement.CONTINUE]:
+                if request.type in [StatementType.BREAK, StatementType.CONTINUE]:
                     loop = self._find_nearest_loop(__stack__)
-                    if request == Statement.BREAK:
+                    if request == StatementType.BREAK:
                         loop._should_break = True  # pylint: disable=protected-access
                     else:
                         loop._should_continue = True  # pylint: disable=protected-access