TextFileOutput improvements

[eed3si9n]

This is a feature proposal

Currently we have TextFileOutput but it has some potential issues.

1. PrintWriter eats exceptions ("never throw I/O exceptions") so it's probably not recommended for anything other than hello world.
2. It's relying on JVM default charset instead of specifying UTF-8.
3. Not buffered.

A quick change would be to make something that just does the above, which is what I added for sbt/serialization#8, but I think we could do better. The problem with buffering is that it needs to be closed afterwards.

Here are some improvement ideas:

1. BufferedWriterOutput — a general Output[String] for java.io.BufferedWriter
2. BufferedWriterOutput.apply function for File that creates java.io.BufferedWriter with UTF-8 hardcoding
3. pickleTo function calls close on the builder it creates (otherwise buffered output would be half hanging)