sg-plos-one

• Add option to specify the critical value for criterion three using the argument sg_crit3_critical_value
• Add option to extract values from multiple measures by specifying a list in the extract_var_list of the function extract_values()
• Internal changes to plot_sg() function to work properly with shiny
• Make output from describe_sg() function clearer
• Create shiny app illustrating main functions of this version, available at https://milanwiedemann.shinyapps.io/shinygains/
• Correct typos
• Add funders to documentation