Add 'ksp url' command
Add a basic built-in 'help' command
Update LATEST for ksp 0.24.2
Display links for mods needing updates when running 'ksp check'
Overhaul version checking and first post detection to be hopefully more robust
Rethink how cacheing works

There is now an explicit cache, in ~/.cache, for mod version info. It is
explicitly refreshed with "ksp refresh"; other commands will populate
the cache on misses but will not update stale entries. This adds another
step to checking for updates (ksp refresh; ksp check) but hugely
speeds up repeated "ksp check" commands and reduces load on the forums.