Allow custom styles

[SimonLab]

by default elm-criteria applies the minimum necessary for styling. The idea is to let the users apply styling themselves. If more styling were applied by default it might limit the user of the package.

So we need to find a way to let the users define/apply the styles easily.

• define custom functions in the configuration which defined a list of attributes which will be added to the main element of the package (button, label, images...)
• create list of classes in the package that users can defined in their stylesheet

[SimonLab]

example of a customised elm-criteria:

[SimonLab]

done with #11 #18