Support indexing of xDai chain

xDai chain has seen some good growth and usage in the past months. Many projects such as Sushi, Omen, Curve, Swapr, DXdao, Honeyswap, Bao and others have chosen xDai due to the almost non-existent transaction fees. With 124m USD in TVL I feel like xDai is a strong candidate, plus, it being EVM compatible should help.