Add --http-header option to spectool
    
    This patch adds a new misc option --http-header. Which can be used multiple
    times to create a list of custom headers to send with the HTTP request
    used to download files (patch or sources).
    
    The form is "header:value" or "header: value".
    
    I have found the need for this when needing to send in a gitlab token
    along with spectool to download sources:
    
      spectool -g ../my.spec  --http-header "PRIVATE-TOKEN: <token>"