Passing additional arguments to underlaying commands
    
    There are two groups of arguments:
     * standard arguments defined by individual parsers
     * extra arguments that relevant for some (individually allowed)
       commands. They are passed to underlying command (usually appended
       to the rest of arguments on commandline)
    This commit adds a common way that allow user to use extra arguments
    for x-pkg commands. These include mockbuild, clone, srpm, local,
    prep, compile, install, verify-files.
    Extra arguments should be placed at the end of the command
    and separated by '--'. Example:
        rhpkg --release eng-rhel-7 srpm --arch amd64 -- --define "'python2 0'"
    
    JIRA: RHELCMP-101
    JIRA: RHELCMP-141
    Fixes: #432
    Fixes: #413
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>