logging: introduce appenders

[DartBot]

This issue was originally filed by @seaneagan

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Currently, we have Logger.onRecord, and you can do something simple like:

logger.onRecord.map((record) => formatRecord(record)).listen(print);

But there is no easy way to log to a file (issue #8686), a remote system (issue #8443), etc.

In Java's logback this is handled by Appenders:

http://logback.qos.ch/manual/appenders.html

Appenders in dart could implement one of the Sink interfaces, possibly StreamSink<String> ? Maybe instead call it LineSink ? LogSink ?

then could do something like:

logger.onRecord.pipe(new ConsoleLineSink());

[madsager]

cc @gramster.
cc @sigmundch.
Added Area-Library, Triaged labels.

[lrhn]

Removed Type-Defect label.
Added Type-Enhancement, Library-Logging labels.

[lrhn]

Removed Area-Library label.
Added Area-Pkg label.

[anders-sandholm]

Removed Library-Logging label.
Added Pkg-Logging label.

[DartBot]

This issue has been moved to dart-lang/core#415.