convert_csv_to_mt.py: Add option to include variable spread.

[kenorb]

If --spread is -1, then try to calculate real/variable spread as per logic in CSV2FXT.mq4

extern bool   UseRealSpread=false; // Use real (variable) spread

This can be checked while running the code by:

Comment(“Spread: ” + DoubleToStr((Ask – Bid) / Point, 2));