Various enhancements to bound helpers, closes #1665 #1768. 

1. Bound helpers now support multiple properties `{{foo prop1 prop2}}` (previously only supported only 1)
1. Helpers declared as bound can be invoked by their unbound former via the `unbound` helper: `{{unbound foo prop1 prop2}}`
1. Hash parameters can now be bound via the `Binding` suffix: `{{foo prop1 thingBinding="whatever"}}`
1. Exposes an array of bound property names via `options.data.properties` (see #1665)