Add a flag for dynamic date range evaluation in patterns.

Add a flag for dynamic date range evaluation in patterns.

By default, Mutt evaluates relative date patterns, such as "~d<1d"
against the date when the pattern is compiled.

For index-format-hook, we need to pattern match against a message
based on the current date/time, so that for example, conditional date
format patterns correctly match after Mutt has been open for many
days/weeks.

Add a flag, MUTT_PATTERN_DYNAMIC, that changes ~d and ~r evaluation to
reevaluate the min/max range with each match.

This will of course be slower, but for the index, where a screenful of
messages at a time are evaluated, is an acceptable tradeoff against
accurate pattern matching.