Add group standings and use ArgParse for country/group/period selection

I've added some improvements on @TMorreale0 's [implementation](https://github.com/fatiherikli/worldcup/pull/19) of showing group standings.

I've also added the ArgParse parsing of the arguments, adding country, group or period arguments into a mutually exclusive group, so that it will be easier to select one of them. 

I'm not very good at formatting the strings and I'm open to any suggestions.