Add support for named placeholders in custom task

'custom' tasks can have named placeholders to be applied to the
'command'
Azure#374

convoy/task_factory.py

@@ -333,7 +333,10 @@ def generate_task(task, storage_settings):
                 args = module.generate()
         for arg in args:
             taskcopy = copy.copy(base_task_copy)
-            taskcopy['command'] = taskcopy['command'].format(*arg)
+            if isinstance(arg, collections.Mapping):
+                taskcopy['command'] = taskcopy['command'].format(**arg)
+            else:
+                taskcopy['command'] = taskcopy['command'].format(*arg)
             yield taskcopy
     elif 'file' in task_factory:
         for file in _get_storage_entities(task_factory, storage_settings):

docs/35-batch-shipyard-task-factory-merge-task.md

@@ -495,8 +495,9 @@ positional argument (i.e., `*args`), we are creating a range from `0` to that
 argument value and `yield`ing the result as a iterable (tuple). Yielding
 the result as an iterable is mandatory as the return value is unpacked and
 applied to the `command`. This allows for multiple parameters to be generated
-and applied for each generated task. An example corresponding configuration
-may be similar to the following:
+and applied for each generated task. The iterable can also be a dictionary to
+allow named placeholders when applied to `command'. An example corresponding
+configuration may be similar to the following:
 
 ```yaml
 task_factory: