Add commands for interacting with Koji side-tag plugin
    
    There is a Koji plugin that can create, list and remove side-tags [0].
    This patch adds support for a commands to do that.
    
    [0] https://pagure.io/sidetag-koji-plugin
    
    It is used like this:
    
        $ rpkg request-side-tag [--base-tag=FOO]
        $ rpkg list-side-tags [--mine|--user=LOGIN] [--base-tag=FOO]
        $ rpkg remove-side-tag TAG
    
    The base tag is used as a parent of the new side tag. If not given, rpkg
    will find build tag of current target and use it.
    
    The plugin creates both tag and target (with the same name), so the
    output of rpkg contains a suggestion on how to submit builds to the new
    target.
    
    Fixes: https://pagure.io/fedpkg/issue/329
    JIRA: COMPOSE-3591
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>