Minimal logging library

Principles:

• https://12factor.net/logs
• single appender, e.g. STDOUT is trivial. (Everything else can be built atop this if you think you need it (but you probably don't))
• functions all the way down
• just Clojure data-structures
• trivial to create appenders (e.g. like Timbre docs claim it is)
• play nicely with pipeline style (like timbre's spy)

Secondary concerns:

• compiling out hidden log-levels with macros
• tracing/line numbers and such
• dynamic reloading of config

Explicitly NOT design concerns

• multiple appenders (just provide a map as your function)
• Java logging (your appender can interface if needed)
• writing to files (>> /var/log/foo.log or add to function)
• logrotation (logrotate.d or add to appender function)
• filtering (grep - or custom valid-function)
• complicated dispatch