[#400] Dynamic logic to infer procedures/hooks arguments.

This allows procedures and event hooks to have more flexible signatures.
Along the lines of:

```python
@when(SomeEvent)
def hook(event):
  ...

@when(SomeOtherEvent)
def hook2():
  ...
```

Instead of supporting only the full context spec:

```python
@when(SomeEvent)
def hook(event, **ctx):
  ...
```

Closes: #400

examples/config/hook.py

@@ -1,9 +1,13 @@
-# A more versatile way to define event hooks than the YAML format of `config.yaml` is through native Python scripts.
-# You can define hooks as simple Python functions that use the `platypush.event.hook.hook` decorator to specify on
-# which event type they should be called, and optionally on which event attribute values.
+# A more versatile way to define event hooks than the YAML format of
+# `config.yaml` is through native Python scripts. You can define hooks as simple
+# Python functions that use the `platypush.event.hook.hook` decorator to specify
+# on which event type they should be called, and optionally on which event
+# attribute values.
 #
-# Event hooks should be stored in Python files under `~/.config/platypush/scripts`. All the functions that use the
-# @when decorator will automatically be discovered and imported as event hooks into the platform at runtime.
+# Event hooks should be stored in Python files under
+# `~/.config/platypush/scripts`. All the functions that use the @when decorator
+# will automatically be discovered and imported as event hooks into the platform
+# at runtime.
 
 # `run` is a utility function that runs a request by name (e.g. `light.hue.on`).
 from platypush import when, run
@@ -16,12 +20,14 @@ from platypush.message.event.assistant import (
 
 
 @when(SpeechRecognizedEvent, phrase='play ${title} by ${artist}')
-def on_music_play_command(event, title=None, artist=None, **context):
+def on_music_play_command(event, title=None, artist=None):
     """
-    This function will be executed when a SpeechRecognizedEvent with `phrase="play the music"` is triggered.
-    `event` contains the event object and `context` any key-value info from the running context.
-    Note that in this specific case we can leverage the token-extraction feature of SpeechRecognizedEvent through
-    ${} that operates on regex-like principles to extract any text that matches the pattern into context variables.
+    This function will be executed when a SpeechRecognizedEvent with
+    `phrase="play the music"` is triggered. `event` contains the event object
+    and `context` any key-value info from the running context. Note that in this
+    specific case we can leverage the token-extraction feature of
+    SpeechRecognizedEvent through ${} that operates on regex-like principles to
+    extract any text that matches the pattern into context variables.
     """
     results = run(
         'music.mpd.search',
@@ -31,16 +37,17 @@ def on_music_play_command(event, title=None, artist=None, **context):
         },
     )
 
-    if results:
+    if results and results[0]:
         run('music.mpd.play', results[0]['file'])
     else:
         run('tts.say', "I can't find any music matching your query")
 
 
 @when(ConversationStartEvent)
-def on_conversation_start(event, **context):
+def on_conversation_start():
     """
-    A simple hook that gets invoked when a new conversation starts with a voice assistant and simply pauses the music
-    to make sure that your speech is properly detected.
+    A simple hook that gets invoked when a new conversation starts with a voice
+    assistant and simply pauses the music to make sure that your speech is
+    properly detected.
     """
     run('music.mpd.pause_if_playing')