Adds new Expansion style

[robertogvieira]

• Adds new ExpansionStyle: singlePerSection;
• Adapts existing code to handle new style.

Notes:

1. This PR is based on PR Expand cells in different sections #53 (where the initial idea for this new case came from);
2. Demo (ExpandableCellDemo) wasn't updated with a new button to test this new expansion style behaviour to avoid "overcrowding" the navigation bar with buttons.
   TL;DR: it can be easily tested by simply replacing the style (ExpansionStyle.single) currently set in method ViewController.expandSingleButtonClicked(_:) with ExpansionStyle.singlePerSection.

[pikaboo]

You need to
if self.expansionStyle == .single { closeAll() }
After you have gotten the index of the cell with
if let cell = self.cellForRow(at: indexPath) {
and before you do this:
if let correctIndexPath = self.indexPath(for: cell) {

because after all the cells close, the index of the cell at hand is not the index it was when it was open.
The same goes for the condition if the style is singlePerSection

[pikaboo]

could you update documentation please ?

[pikaboo]

@younatics Can you release new pod version? I'm not able to

[younatics]

@pikaboo I will do :)