Allow the usage of environment variables in the external command configuration

It would be easier if it was possible to specify the external command and arguments with environment variables.

E.G. the settings in https://greenshot.atlassian.net/browse/FEATURE-943#icft=FEATURE-943 are:

1. Command

   C:\WINDOWS\System32\rundll32.exe

2. Arguments

   "C:\Program Files\Windows Photo Viewer\PhotoViewer.dll", ImageView_Fullscreen {0}

After implementing this feature they would be:

1. Command

   %SystemRoot%\System32\rundll32.exe

2. Arguments

   "%ProgramFiles%\Windows Photo Viewer\PhotoViewer.dll", ImageView_Fullscreen {0}

Which should make it more portable.