Append data to environment variables

Hi there,

for one of my projects I need to add some paths to PATH. Therefor I enhanced the existing implementation for setting environment variables.

Notable changes to previous implementation:
* If one changes the same environment variable multiple times in one scenario only the first change will be recorded and can be restored (I thought that makes more sense than restoring only the previous version)
* Add column for action: One can now tell aruba to append data by using `+` or `append` in action column. This column is optional.