Add support for tag/target macros for Mageia
    
    Discussions with Son_Goku (from Mageia) on IRC revealed that they need
    per tag/target macro support in Koji. We store tag macros in 'Extra Options'
    and macro has to start with '%'.
    
    Example:
    
        % koji taginfo f29
        Tag: f29 [11]
        Arches: riscv64
        Groups:
        Required permission: 'admin'
        Tag options:
          %mymacro : 'value123'
          %packager : 'Fedora Project RISC-V'
          mock.package_manager : 'dnf'
        [..]
    
    Tag macros can be added using koji edit-tag.
    
    The tag macros are allowed to overwrite macros set in koji.genMockConfig.
    For example, %packager is set in koji.genMockConfig by default. The mock
    configuration generated by kojid was:
    
        [..]
        config_opts['macros']['%_host'] = 'riscv64-koji-linux-gnu'
        config_opts['macros']['%mymacro'] = 'value123'
        config_opts['macros']['%_host_cpu'] = 'riscv64'
        config_opts['macros']['%vendor'] = 'Fedora Project'
        config_opts['macros']['%distribution'] = 'Fedora Project'
        config_opts['macros']['%_topdir'] = '/builddir/build'
        config_opts['macros']['%_rpmfilename'] = '%%{NAME}-%%{VERSION}-%%{RELEASE}.%%{ARCH}.rpm'
        config_opts['macros']['%packager'] = 'Fedora Project RISC-V'
        [..]
    
    This two-line change was tested on Fedora RISC-V koji instance.
    
    Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>