Split into sub commands

[gwillem]

Currently we accept multi path arguments to support the --merge command, which is a bit hacky. Should probably refactor to use (for example) github.com/spf13/cobra and then have commands:

• scan
• add
• merge
• strip (to deduplicate X given main db Y, useful to store delta hashes per release)
• test (to see if a particular filename or line is in the db)