CLI Command that generates all flavors with single flag

[vlazdra]

Hey @jonbhanson I've completed the extension of the package to add the ability to provide a single flag that would scan through the root directory and using regex find the files that should be used to generate the splash screens. Closes #751

I might have changed more than it was needed, but since I was here. 😄

Here's a list:

• Made an enum for the options so to minimise potential mistakes
• Added a help option and description for all commands
• Added the --all-flavors command to generate all of the configs
• Added a helper util file/class for validating the flavor setup and file matching
• Added tests to validate the new flavor changes and validations
• Updated the CHANGELOG.md with the new feature release
• Updated the README.md with the information on how to use the new command

Please take a look at the changes, and let me know if you have anything you'd wish I change.