accdb: use variables for `tokens[0, 1:]`
accdb: filter: understand item numbers directly
accdb: implement range search as a filter

Alright, this will be slow, but so are all other filters already... I'll
optimize later, with some indexing. Not yet a problem with 500 entries.
Merge branch 'accdb/range-filter'

Teach search filter the item range syntax. This will allow subcommands
like 'show' or 'tag' to be moved from expand_range() to the full search
syntax, e.g. `accdb tag @!2fa.oath.psk +2fa` (admittedly not a very
useful example).