Tags for packages

[ZookaOnGit]

Add tags to packages.

Tasks:

• update reindex.lua to create mpkg.packages.json with new tags element
• update mpkg show to show tags
• update mpkg search (add links to click to search sister tags)
• update Mudlet package exporter to support tags element
• notify devs about above change for 3rd party support (i.e., de/muddler)
• update website for this new feature

Redesign of Package Exporter may be imminent to support additional features; 7051