no need to use mission_slug parameter when get-git is using

[oduvan]

$ checkio-cli get-git https://github.com/Checkio-Game-Missions/checkio-empire-striped-words striped-words

in that case adding a striped-words is unnecessary because this slug can be gotten from url.

config file should contain all prefix in git url that can be used for getting slug.

GIT_URL_SLUG = ['checkio-empire-', 'checkio-']