Sheet size configuration option

• Add sheetSize option in NBBottomSheetConfiguration

Background view color configuration option

• Add backgroundViewColor option in NBBottomSheetConfiguration
• Add support for dark mode in Example project

Swift 5

• Add UI tests in Example project
• Convert library to Swift 5

Improve podspec

• Improve content of podspec file

Update Example

• Use a table view to demonstrate the component in the Example project

Initial release

Initial release of NBBottomSheet.