Created a multi writer option which logs to both syslog and files

Merging changed.
For multi, I propose that the contract for Write is that it will attempt to each Writer.Write, but will ignore errors. And based on that, we should just eat error internally. This also means we should always return `len(b)` and just ditch `n`.

Thoughts? There are no delivery guarantees in logshifter by design, so seems appropriate here.