Log Warnings at Warning level by default

[Ethan Stern]

When I ask “Log If Error” to include warnings, I would like it to log them as Warning, not as Errors. (I’m not sure I would ever really want to automatically upgrade their severity to Error.)

[[e-mail]]

Added buffered log sink and test coverage, resolved issue #1, resolved issue #2, resolved issue #3, resolved issue #4, GPackage version 0.7.0

→ <<cset d4a062399a59>>