Release version 0.5.0

Update from log 0.3 to log 0.4. Both log versions are interoperable, but line numbers from libraries using 0.4 won't show up in binaries with recievers using log 0.4.
- To clarify: both fern 0.4 and 0.5 will work perfectly with all libraries, but line numbers will always be 0 if you use fern 0.4 and log 0.4.
Remove fern::FernLog. With log 0.4, log records can be constructed directly, and fern loggers can now recieve just a record rather than a record and the formatted display string.
Notable changes in the log crate: log::LogLevel is renamed to log::Level, and log::LogLevelFilter is renamed to log::LevelFilter.
fern::color::ColoredLogLevelConfig has been renamed to fern::color::ColoredLevelConfig to match log crate renamings.
fern tests have been greatly simplified with the new support for creating log records manually. it's now possible to just run "cargo test" and test all of fern's functionality.

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