Add support for the check command

[Johannes11833]

Fixes #57: Adds support for the check command.

• A tuple of a bool and a list is returned. The bool is true if source and dest match.
• The list contains a symbol and all file paths in both directories. The following symbols are used:
  • "=" path means path was found in source and destination and was identical
  • "-" path means path was missing on the source, so only in the destination
  • "+" path means path was missing on the destination, so only in the source
  • "*" path means path was present in source and destination but different.
  • "!" path means there was an error reading or hashing the source or dest.