Interpret string attribute values

[cressie176]

Keep the existing behaviour...

{
   _task: '@some.value.in.context'
}

Deprecate

{
   _task: {
     template: '{{some.value.in.context}}'
}

In favour of

{
   _task: '{{some.value.in.context}}'
}

And in addition support

{
   _task: '[hbs]{{some.value.in.context}}[/hbs] = [eval]1 + 2[/eval]'
}

Where:

• hbs identifies a handlebars template
• eval will invoke javascript eval

These should be extendable to include new tags / processors.

Do not interpret attribute names beginning with np_ e.g. np_message and reserved attribute names with np e.g. _np_task