[x] Create new sub command called set-region and accept no arguments/flags
[x] When called, show list of all available aws regions using termui list (similar to set or export). Once selected, set only region key in ~/.aws/configdefault profile
[x] Handle case when user cancels (Ctrl + C) in the middle of selection
Related discussion: https://github.com/hpcsc/aws-profile/issues/6
set-regionand accept no arguments/flagssetorexport). Once selected, set onlyregionkey in~/.aws/configdefaultprofile