Highlight specific lines
    
    When a specific line in a file is linked, highlight it.
    
    This patch also adds the ability to highlight a line range. Regular
    click on line number always selects that one line. Shift+Left click
    allows selecting a range.
    
    Pagure issue: #528
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>