Support for UICollectionViewDelegateFlowLayout

[cd5z]

add configureCellSize for custom cell size

usage:

        let dataSource = RxCollectionViewSectionedReloadDataSource<<#SectionModelType#>>(
            configureCell: { (datasource, collectionView, indexPath, item) in
                return UICollectionViewCell()
            },
            configureCellSize: { (dataSource, collectionView, indexPath, item) in
                return CGSize(width: 100, height: 100)
            }
        )

[rxswiftcommunity]

	Warnings
⚠️	It looks like code was changed without adding anything to the Changelog. If this is a trivial PR that doesn't need a changelog, add #trivial to the PR title or body.

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[freemansion]

@kzaher any plans to merge this PR in the nearest time?

[lordzsolt]

@freemansion Shouldn't this go into RxCocoa?