Allow Call Render Function in Objects

Allow to call functions on objects result.
```go
{% v.Obj().RenderHeader(123) %} // Header is a render function generated by quicktemplate (so the 3 variants of the func are available)
```
Currently this is not possible at all. That makes it quiet unflexible to use render functions on objects.
```go
v.StreamRenObj()....
```
