Optionally display list of columns that are not documented in doc coverage report

[Thootje]

Hi! Really like this neat tool to add simple coverage reports to our CI process.

The only thing I'm missing is to optionally display the columns that are not documented at the compute check. This would provide feedback to the developer with which columns are missing. In our usecase, we want to enforce documentation on every column since we are building data products that are used my multiple people and want to guarantee documentation.

By having a list of non documented columns per model, a developer can easily see which columns need some extra work.