Add Prefix to Cache Keys
I think the keys should be prefixed to not conflict with other keys.
This adds a configurable key prefix defaulted to 'vmenu:' to all
cache keys. It can be overridden in the config file but I am not
adding it by default because I think it would just cause more confusion
than necessary.
Cache Tag and Recipe Lists
Because of the slowness of the Evernote API and the throttling limits,
I am going to cache big batches of data. To start with, this change
caches the resulting tag list and recipe list for a given tag.