Enhance log experience

[killercup]

In #30, we landed initial support for being a log target. But we can go further.

Ideas:

• Add custom log macros (trace, info, debug, warn, error) that take a (mutable ref to an) Output as first param
  • Behind a cargo feature called "log-macros" (or similar)
• Be compatible with slog and support its many adapters
  • Behind a cargo feature called "slog"
• Look at https://github.com/rust-lang-nursery/log/pull/296 -- structured data in log macros