Add task metadata

Metadata is supported in the config file on a per-task basis. It is to be put
in the 'meta' key's value json.

Currently 'title' and 'description' keys are supported. See the example below.

```
[
    {
        "meta" : {
            "title" : "This is the title",
            "description" : "This is a longer description, not shown with -Q"
        }
    }
]
```

dotbot/dispatcher.py

@@ -22,7 +22,11 @@ class Dispatcher(object):
     def dispatch(self, tasks):
         success = True
         for task in tasks:
+
             for action in task:
+                if action == 'meta':
+                    self._handle_metadata(task['meta'])
+                    continue
                 handled = False
                 for plugin in self._plugins:
                     if plugin.can_handle(action):
@@ -42,5 +46,12 @@ class Dispatcher(object):
         self._plugins = [plugin(self._base_directory)
             for plugin in Executor.__subclasses__()]
 
+    def _handle_metadata(self, metadata):
+        if 'title' in metadata:
+            self._log.warning(metadata['title'])
+
+        if 'description' in metadata:
+            self._log.info(' ' + metadata['description'])
+
 class DispatchError(Exception):
     pass

dotbot/executor/__init__.py

@@ -1,3 +1,4 @@
 from .executor import Executor
 from .linker import Linker
 from .commandrunner import CommandRunner
+from .meta import Meta

dotbot/executor/meta.py

@@ -0,0 +1,16 @@
+from . import Executor
+
+class Meta(Executor):
+    '''
+    Dummy handler for metadata support
+    '''
+
+    _directive = 'meta'
+
+    def can_handle(self, directive):
+        return directive == self._directive
+
+    def handle(self, directive, data):
+        if directive != self._directive:
+            raise ValueError('Header cannot handle directive %s' % directive)
+        return True