CHANGELOG incompatibility with GPM

[w00fz]

Hello @finanalyst,

I have noticed in our GPM logs that your plugin's CHANGELOG is not compatible with GPM. Our GPM is quite sensible to the way the CHANGELOG is structured and it does require consistency. This PR should fix the problem for you and I also wanted to highlight what type of changes I have made for you to know in the future.

1. Sections: We only support new, bugfix and improved as sections of a changelog. Would be great to support some of the ones you came up with such as documentation, minor, change but at this time we do not support those.
   Also, don't group multiple entires of the section name. Only 1 [](#bugfix) per version should be present.
2. Date Format: The date format we support is only US or EU in the short numeric notation. For US, today would be 12/23/2020 for EU today's format would be 23-12-2020. I have updated all your dates to reflect that.
   Also, the date is quite important to be just a date, the format < 28 April 2018 is not supported. I did set that date for you to 31-01-2018 which appears to be your very first commit on the repository.
3. Version Format: Although a range of versions is ok and will work (# v1.0.0 - v1.2.0), CHANGELOGs are supposed to be the journal of each singular version. Particularly in Grav we have a feature that allows to display the CHANGELOG of a resource from the current version installed to the latest available. This would not work if the version definition was in range format.