Added -g:auto and -g:off options to gameplay-encoder

As the title says I was trying to automate my asset pipeline and was having trouble because of the prompt to group animations. I wanted to enable the option from the command line. To address this issue I have added two new command line options -g:auto and -g:off. If either option is set then the encoder does not prompt. If neither option is set then the encoder will prompt as normal.