tagging documents

[mark-ep]

implement system for tagging documents

some command to set/view tags, perhaps:

doclib document tag PROJECT CATEGORY DOCUMENT
doclib category tag PROJECT CATEGORY

to view tags on category/docs

doclib document tag PROJECT CATEGORY DOCUMENT [TAG1 TAG2 ...]
doclib category tag PROJECT CATEGORY [TAG1 TAG2 ...]

to tag documents/category
also need to store/retrieve tags from somewhere