Make .night_mode CSS class name customizable?

[krassowski]

It seems that some users would prefer to have the CSS class to be .nightMode instead of .night_mode.

While I am not usually happy with following the arbitrary design changes made upstream without consulting the more popular downstream alterantives (allusion to our .night_mode being there a bit longer than recently built-in .nightMode), it was brought to my attention (thank you Hector) that some users may prefer to have the class name the same for their IPhone app and this add-on.

However, such change would both break backwards compatibility and introduce potential conflicts of the two nightmode mechanisms (the recent, very simple bulit-in feature of Anki and this add-on). Instead, let's have a seting which allows users to change the class name - with them taking the responsibility for potential issues.