Create a logic to handle primary and secondary buttons on atom-steps-modal and new property for initial step

[jowjow22]

1 - Replicate the same logic used to change the titles of modal on each step, to allow user to only give an array of strings to be rendered
2 - Create a new property that allows the user to set the step that should be the initial one, even if it has other steps rendered behind that initial

[tassioFront]

@jowjow22, is it done?