Option to change the ".alert" class

[deflox]

As far as I can see there's no option to change the default ".alert" class for a single message.

It would be useful to add an option to modify this class, since I'm actually working with bootstrap and customization of the ".alert" class also overrides the bootstrap alert boxes styling.

I would make the ".alert" class the default option if the user does not define the class.

[mdennisa]

@deflox I have the same problem working with custom alert classname since its conflicting with bootstrap 4.

I forked and make some adjustment along with css example here https://github.com/mdennisa/ohSnap

[deflox]

Thanks! I will take a look at it.