Add support for documented --blur-* options

Handle parsing of `--blur-method`, `--blur-size` and `--blur-deviation`
in config file and as command line options as documented in the manpage.

src/config.c

@@ -540,6 +540,8 @@ char *parse_config(options_t *opt, const char *config_file, bool *shadow_enable,
 	    .detect_client_opacity = false,
 
 	    .blur_method = BLUR_METHOD_NONE,
+	    .blur_radius = -1,
+	    .blur_deviation = 0.84089642,
 	    .blur_background_frame = false,
 	    .blur_background_fixed = false,
 	    .blur_background_blacklist = NULL,