[Freqtrade] Implement Pairs Trade Opening Priority

## Describe your environment (if applicable) * Operating system: ____ * Python Version: _____ (`python -V`) * CCXT version: _____ (`pip freeze | grep ccxt`) * Freqtrade Version: ____ (`freqtrade -V` or `docker-compose run --rm freqtrade -V` for Freqtrade running in docker) ## Describe the enhancement Sometimes it could be useful to order pairs **after** dataframe analysis for each pair is done (probably based on some indicator or whatever). Let's imagine that target indicator is "probability of profit" for a pair during the following K candles, for instance predicted by some Machine Learning technique. Someone may want to open trades in max-to-min predicted probability order to maximize expected wins / by expected return based on predicted probability and position size to maximize expected capital growth / whatever else. Currently there is no **clear way** to provide "priority" information out of the strategy so the bot will know which pair to enter first.