Provide special pre- and post-actions for steps with sub-steps

[pvandervelde]

Some steps execute multiple sub-steps. It would be useful if users could execute pre- and post-step actions for the sub-steps as well.
The following steps should have sub-step pre- and post-actions

• vcs merge
• build.binaries
• archive pack
• nuget pack
• generate files (build & test)
• update assemblyinfo
• push to directory
• push to git branch
• push to github
• push to git repository

[pvandervelde]

This turns out to be required for the generation of the assembly info files as well. Currently it doesn't generate the correct configuration information.