Use io.Writer instead of []byte in Render() method

[frederikhors]

Discussing with the great developer @valyala in thread valyala/quicktemplate#58 about the amazing Authboss he gave us a suggestion to optimize a bit: valyala/quicktemplate#58 (comment)

As for the performance ... superfluous memory allocation when returning the byte slice from bytes.Buffer. The memory allocation could be avoided if Render() could accept io.Writer to write template output to.`

Can we just use io.Writer instead of []byte in Render() method?

[aarondl]

Although he's correct, the main reason for this to exist is to enable a JSON renderer which doesn't accept an io.Writer anywhere. I'm really not opposed to the change, but it would have to be a RenderWriter interface we upgrade to from a Renderer otherwise it'd break backwards compatability. I'd accept a PR for this, will not implement it myself.