Factor out ctor+fields generation into a single _structbody template
refactor common templates into common.st
swift-generator formatting fixes
Make generator usable from the command line
Move generator main class to new module: swift-generator-cli
Fix dependencies for swift-load-generator-cli
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()
Clean up command-line tweak options

Remove default ADD_CLOSEABLE_INTERFACE tweak. If this tweak is on by default, it can't be turned off, so I'll remove it from the defaults.

And remove specialized -add_thrift_excpetions switch, since it is accessible as a tweak.
Fix JCommander complaining that parameter should be List not Iterable
Use ImmutableLists for mapping from generator config to template
Replace nested Object.firstNonNull with more readable condition blocks
Remove unused 'group ...' header inputstreamreader constant
Fix generator indentation for method declarations
Code formatting cleanup and remove some unused imports