Support for custom completers
    
    Argcomplete library supports custom completers, but their assigning
    to arguments should be done before rpkg initialization. This commit
    introduces mechanism how to pass specific completers for rhpkg/fedpkg
    to rpkg.
    
    JIRA: RHELCMP-89
    
    Signed-off-by: Dominik Rumian <drumian@redhat.com>