-debugMode: Enable executing targets after the initial build

[sayedihashimi]

When Invoke-MSBuild is called with the -debugMode flag we return the resulting ProjectInstance object. Users can use that object to then execute specific build targets. We could do the following to help here.

• Add a function on the build result object to call build
• Add a parameter to Invoke-MSBuild to accept a ProjectInstance (available only under -debugMode parameter set)