Load theme when `theme` parameter is 'semantic-ui'.

def devpiserver_cmdline_run(xom):
    '''
    Load theme when `theme` parameter is 'semantic-ui'.
    '''
    if xom.config.args.theme == 'semantic-ui':
        xom.config.args.theme = resource_filename('devpi_semantic_ui', '')
        xom.log.info("Semantic UI Theme loaded")