Possible to set/add log-file "appender"(?) on the fly?

[hoijui]

I tried for quite a while to get this done with a reload layer, similar to #2121,
without success.
I though I might be able to add logging to io::sink *which implements Writer),
and later change that writer to the result of a File::open("log.txt")?.
Am also happy about less hacky solutions though.

I just want to initilaize logging to stderr first, and later, potentially, also add logging to a file.

[davidbarsky]

I don't think creating a MakeWriter than can switch between stderr and then writing to a file is hacky or unreasonable—it's why that interface exists.