Add --nopush option to the build

[CharliePoole]

This would be extremely handy in testing out parts of the build, which normally impact the user, like publishing packages and doing the release on GitHub.

[CharliePoole]

I forgot that cake already has a --dryrun optiion, which doesn't run any targets, just lists what would be run.

What I want is to run the tasks to test the logic but not actually push anything. So --nopush should keep us from pushing to myget, nuget or chocolatey or creating a release on github. At the same time, it should list what would be done given the current settings, etc.

[CharliePoole]

This issue has been resolved in version 3.13.2

The release is available on:
GitHub.
NuGet packages are also available NuGet.org and
Chocolatey Packages may be found at Chocolatey.org