Enhancements for convert from nuget command 

Includes --no-install and --force options.
The --force flag added some complexicity. To make my life easier, I output a warning and ignore the package if someone forces convert-from-nuget, having a package in packages.config that is already included in dependencies or references files. 
When forcing the command, new packages from packages.config are appended to the end of dependencies/references files.
@bartelink could you please test again the comand for your cases?
I also added support for reading custom package sources from a nuget.config file. After reading from the nuget.config file, I remove it, making assumption that it's not included in any solution/project.
should be `safeDeleteFile configFile.FullName` right?