Allow multiple anonymisation scripts at runtime

[rkm]

Currently, only a single script is supported by the -a / --anon-script option. We need to support multiple scripts without restarting the service. This could be accomplished by:

• Allowing the -a argument also to support use of a directory
• When a directory is passed, all scripts inside the directory are loaded and registered
  • They could be registered by filename or checksum
• When in this mode, requests to anonymise a file must also specify the script to use, returning an appropriate error if the script is unspecified or missing

[rkm]

@jas88 Does the above implementation look ok? Would you like to have a go at implementing it?