Check that the category attribute contains only a single slug

[ggrossetie]

To move forward with #35 we could also check that the value is one of:

• operations
• cypher
• cluster
• installation
• import-export
• drivers
• browser
• development
• tools
• desktop
• performance
• security
• server
• bloom
• support
• geospatial

If not, we could issue a warning/notice.