[Feature Request] Support indexes

[vityav]

Currently when attempting to retrieve an index (^DJI, ^GSPC, ^IXIC, etc) the program throws a key error about yfcd.exchange_locks[exchange] inside setting the time zone. Ideally this would be able to pull and cache those values too. Yahoo has them listed with open/close history which is often useful for data analysis and comparisons to market trends.