Ability to set repeating group delimiter dynamically by using first tag of the first group

[andreydp]

Hello

In some cases and especially drop-copy flows we need to aggregate multuple sources traffic into a single FIX messages flow. For this purpose we have a superset FIX dictionary. The only issue we have - repeating groups delimiter, as by default it's always taken from the dictionary (first tag from the group).
I suggest of having a new property which will allow to dynamically set the repeating group delimiter tag, based on the actual FIX message first group tag.
I've already made this enhancement and added some tests, FYI: #674