text: Title: Retrieving crypto data from complementary data aggregation services / Coingecko?
  Body: Hi,
  
  Currently, cryptofeed is managing data coming from exchanges.
  Please, is there any contraindication for proposing new data feeds like CoinMarketCap, CoinGecko, Nomics...
  From these data feeds, I would like to get complementary 'global' crypto data like circulating supply, global trading volumes, dominance...
  
  Could related PRs be accepted in cryptofeed?
  If yes, what would be your recommandations?
  I see 3 main points:
   - switching from 'exchange' to 'feed' or creating a new type of feed like 'aggregator' (I come from cryptostore, and did not start to have a look into cryptofeed sources. I am referring here to a choice of words :)).
   - introducing 'single' asset (not pairs)
   - new data types: `circulating_supply`, `trading_volume`, `dominance`...
  
  Thanks for your feedbacks.
  Have a good day,
  Bests,