Make snapHeight an opt-in feature

[joelmukuthu]

• [x] do not set the height of the element
• [x] do not set the height of the child elements
• [x] get the element's new scrollTop from the next child's offsetTop see 7fb11cc46542078427d77637f60f93af2b66e506
• [x] remove watchSnapHeight
• [x] remove snap-height attribute
• [x] remove snapscroll's controller that is only needed needed to update snapHeight
• [x] remove fit-window-height directive
• [x] extra: separate fit-window-height into a separate module other modules already exist
• [ ] publish as breaking changes
• [ ] update demo

So that the height of the snapscroll element or the children can be controlled fully in CSS.

[joelmukuthu]

Decided to make snap-height an opt in feature instead i.e. only change the height of the snapscroll element AND it's children if the snap-height attribute is set