[enh] overwrite secret_key from env with the SEARX_SECRET env var if exists - closes #845

searx/__init__.py

     logger.warning('You are using an old openssl version({0}), please upgrade above 1.0.2!'.format(OPENSSL_VERSION))
 
 logger.info('Initialisation done')
+
+if 'SEARX_SECRET' in environ:
+    settings['server']['secret_key'] = environ['SEARX_SECRET']