def ultimateReplace(app, docname, source):
    result = source[0]
    for key in app.config.ultimate_replacements:
        result = result.replace(key, app.config.ultimate_replacements[key])
    source[0] = result

def setup(app):
    app.add_config_value('ultimate_replacements', {}, True)
    app.connect('source-read', ultimateReplace)