Release version 0.4.0

• Rework API surface to be builder-based for simpler configuration

  • Rename DispatchConfig to Dispatch, OutputConfig to Output and FernLogger to FernLog

• Rework inner log structure for more efficiency

  • Different outputs are now stored in an enum rather than every sublogger being a Box with dynamic dispatch
  • Remove LogError; handle errors within individual loggers now - and only within loggers which actually need it
  • Remove unnecessary wrapping of streams with an Arc (now just uses Mutex for File streams)
  • Remove unnecessary wrapping of Stdout and Stderr streams with a Mutex, when they are already synchronized
  • Pass around just &fmt::Arguments + &log::LogRecord instead of passing each individual LogRecord part

• Move opening of files and stdout/stderr from configuration "building" to configuring

  • Instead of taking OpenOptions, log configuration now just takes an already-opened std::io::File object
  • fern::InitError is now a convenience API, and is never returned from any fern APIs

• Redo formatting to work without allocation - formatting closures now finish with a callback rather than returning a value

• Update examples to use chrono instead of the time crate

  • This removes another extra allocation - chrono can format time directly to a writer, without allocating intermediate the result to a String

• Add much more documentation: almost every builder method has a full example, and all features should be thoroughly explained

• Add appveyor and travis-ci badges to README and Cargo.toml