refactored api check
if -S is used with -U branch is not written configuration file

create if not exist

compare breaks if mirrors.json does not exist eg. chroot condition

only if usr dir exist filecmp is used to compare data files

saves mirrors.json in case it is not existing
the filecmp would break execution if mirrors.json is missing

added exit when using -U since updating mirrorlist fails on an empty chroot
updated docs

fixed typo

updated docs

refactoring code to match #105
updated pot file
updated docs/man
updated CHANGELOG

updated texts