Support for multiple nuget projects

Multiple NuGet projects support

Users can specify several NuGet sections in the config file with the same structure, then for each section, a separate pair of build+publish jobs will be generated.
Job name format for NuGet build and publish now includes slug from the nuget item section.

Better dependency support for nuget

Nuget section have a new optional field publish_after. It is an array of job names than should be finished before publishing the nuget package.

Backward compatibility

This release is backward-compatible, e.g. producing the same workflow for old nuget sections.
nuget config section can be an array, or a map (old behavior). A map will be treated as an array with single NuGet section.
If publish_after field is missing, default dependencies would be generated