Allow for users to categorise secrets

[oubiwann]

E.g., "Personal", "Work", "Company X", "Firm Y", or even if they want to keep browsers separated.

Tasks:

• [x] Add category field to Metadata
• [x] Add --category flag to list subcommand (only show)
• [ ] Add --category flag to export subcommand (only export)
• [ ] Add --category flag to import subcommand (only import)
• [x] Add show categories subcommand (with show cats alias)

Note for more general, flatter, one-to-many "categorisation", flags will be offered:

• 67

[oubiwann]

Gonna save the filtering for #71 and #72 ...

[oubiwann]

Done.