Tasks should support and _args attribute

[cressie176]

{
   _task: 'set',
   _args: ['foo', '@bar']
}

_args should be auto injected into the activity.
An activity shouldn't be able to mix _args and named attributes, e.g. the following should be illegal

{
   _task: 'set',
   _args: ['foo', '@bar'],
   name: 'xxx'
}