added method for FormatLogger to write directly to file and some doc

[ExpandingMan]

For a moment I got myself pretty confused why I could not compose FileLogger and FormatLogger.

This PR explains that to write to a file in a specific format, you should use FormatLogger not FileLogger and adds a method which takes a filename instead of an IO.

In the long run, FileLogger probably ought to be deprecated in favor of FormatLogger and some reasonable defaults.

[fredrikekre]

Thanks, I have been thinking about this change for quite some time too.

[oxinabox]

I kind of what to write a IO subtype that is FastFlushing<:IO that wraps another IO and overloads write to always call flush.
Which would then deprecate the FileLogger entirely infavor of SimpleLogger(FastFlushing(open("myfile.log", "a")))
and would deprecate the always_flush argument from FormatLogger.
However that requires the user opening the stream themselves.

The relevance to this PR is that this this pushes the other way.
Making the constructor take the same arguments as open and just pass them on.
Is there a particular reason top prefer
FormatLogger("myrecord.log") over FormatLogger(open("myrecord.log", "a"))?
It is a bit shorter that is for sure
OTOH FormatLogger("myrecord.log", append=false) is similar in length to FormatLogger(open("myrecord.log", "w")).

I don't have super strong opinions about this though, (or any other logging sinks.)
So if @fredrikekre thinks this is good, then i am not going to block it.

@fredrikekre you should merge this, yes?