Address review feedback

- Changed TemplateLoader to take a set of files instead of one, and thus got rid of the implied common.st template
- Changed reference to 'mutable' types in CLI param names to 'beans'
- Removed code-based configuration of logback and added an xml config
Remove getInputs() from SwiftGeneratorConfig

Based on review feedback that inputs is not a config. Instead, just pass a parameter to SwiftGenerator.parse()