make default theme configurable

I am using kaminari with some custom themes. I felt it is lousy to write `:theme => :my_theme_1` in options every time in views, and it's troublesome when I want to replace my theme from one to another, I think it can be more convenient to set default theme in config file like this:

    Kaminari.configure do |config|
      config.theme = :custom_theme
    end

What do you think?