Changelog for co-log-0.3.0.0

Changelog

co-log uses PVP Versioning. The changelog is available on GitHub.

#77: Important: Use chronos time formatter. This is a breaking change because default field map in RichMessage now contains different type representing time. If you use your custom formatter for time, you should change it. Othwerwise no observable differences in the library API usage will be noticed.
#103: Breaking change: make Message data type polymorhic over the type of severity.

Migration guide: this change is done in backwards-compatible way. If you use any fields of the Message data type, you should rename them according to the following scheme:
```
messageSeverity -> msgSeverity
messageStack    -> msgStack
messageText     -> msgText
```
Export more formatting functions to make implementation of custom formatters easier.
#96: Add simpleMessageAction and richMessageAction to work with Messages.
Use co-log-core of version 0.2.0.0.

#45: Introduce approach for concurrent log writing.
#46: Moves logStringStdout, logStringStderr, logStringHandle, withLogStringFile from Colog.Actions to Colog.Core.IO
#77: Remove relude from dependencies. Add HLint check to Travis CI.
#64: Introduce basic benchmarks.
#20: Add experimental support for logger rotation (see Colog.Rotation module).
#39: Support GHC-8.2.2 and GHC-8.6.2.

#37: Add bounds to all dependencies. Move Prelude to the other-modules section.