2.0.0: Merge pull request #9 from caseycrogers/v2

• Add optional scaleFactor parameter
• Introduce constructors on Gutter: Gutter.tiny, Gutter.small, Gutter.medium, Gutter.large, Gutter.extraLarge
• Extend GutterConfiguration to allow customizing the default gutter sizes and relative scales
• Migrate to flutter_adaptive_scaffold to remove deprecated package