utils: use pure-go tar-gz
ctx: explicit error handling
ctx: impl. AddRepo, RemoveRepo, ListRepos
all: add repo-add, repo-ls and repo-rm subcommands
all: proper handling of local/remote repos and of 'file://' protocol
doc: add doc for repo-add, repo-ls and repo-rm
ctx: handle duplicate repo names
ctx: handle non-existant repo removal
utils: cleanup