Put back YieldTemplate

[SVyatoslavG]

I put back YieldTemplate which was removed earlier. I am using it in my code to "insert" template files. Please put it back. Thank you very much in advance.

[sauerbraten]

How is this different from t, err := set.GetTemplate("foo") and then t.Execute(buffer, nil, context)? Of course you will have to pass the output writer around outside of the set, but I think the new API is more flexible for rendering single templates.