Pass children to templates onUpdate function in second arg

[fewkz]

We should probably do the same thing for templates, by passing a second arg to the onUpdate callback functions with the children

Originally posted by @fewkz in #4 (comment)

[fewkz]

i'm so confused how would this would even work for templates... they are instance-based, would the "children" be instances or elements? idk why I made this issue.
The way I could see this working is it passes the elements to the onUpdate function, and the template would be responsible for doing froact.Roact.mount, froact.Roact.update, and froact.Roact.unmount.