Add ability to "jake link-vs" & "jake unlink-vs" which adds/removes symlink

- Using an elevated command prompt or PS, allows to symlink current build of compiler & services used by Visual Studio. for #1110 

> Create symlinks
 jake link-vs

> Remove symlinks & restore original Visual Studio files 
jake unlink-vs

> Also improved 'jake diff' so it works from PowerShell + looks up WinMerge by defaut
jake diff