Add an option to render not recognized languages without highlight

[fpohtmeh]

The default behaviour for the not-defined languages is the error: [error] Failed to detect the language

The user experience will be improved if he gets the warning and the generated picture instead of the error. The warning should notify about the "Not detected language".

The proposal is to add the option to allow non-highlighted output for unrecognized languages or make the behaviour mentioned the default.