Recommended changes resulting from automated audit

We performed an automated audit of your Cake addin and found that it does not follow all the best practices.

We encourage you to make the following modifications:

- [ ] You are currently referencing Cake.Core 0.26.1. Please upgrade to 0.28.0
- [ ] You are currently referencing Cake.Common 0.26.1. Please upgrade to 0.28.0
- [ ] The Cake.Core reference should be private. Specifically, your addin's `.csproj` should have a line similar to this: ``
- [ ] The Cake.Common reference should be private. Specifically, your addin's `.csproj` should have a line similar to this: ``
- [ ] Your addin should target netstandard2.0. Please note that there is no need to multi-target, netstandard2.0 is sufficient.
- [ ] The nuget package for your addin should use the cake-contrib icon. Specifically, your addin's `.csproj` should have a line like this: `https://cdn.rawgit.com/cake-contrib/graphics/a5cf0f881c390650144b2243ae551d5b9f836196/png/cake-contrib-medium.png`.

Apologies if this is already being worked on, or if there are existing open issues, this issue was created based on what is currently published for this package on NuGet.org and in the project on github.

Start with the addin's .csproj and compare its Cake.Core and Cake.Common references, target framework, and package metadata with the checklist. Update both dependencies to 0.28.0, mark them private, target netstandard2.0, and add the specified cake-contrib icon URL. Done means every audit checkbox is satisfied.

Written by the indexing model from the issue text.