Support `next/last <day of week>`

[dcastro]

Clarification and motivation

We should support references like "next Monday" or "last Tuesday". Currently, we only parse the day of week, but don't consider the "next"/"last" qualifier.

λ> parseTimeRefs "next Monday at 10pm"
[ TimeReference
    { trText = "Monday at 10pm"
    , trTimeOfDay = 22:00:00
    , trDateRef = Just ( DayOfWeekRef Monday )
    , trLocationRef = Nothing
    }
]