Store historical prices from sources that provide CSV files.

Maintaining a database of prices from specific exchanges is difficult or infeasible because exchanges are rate limited so downloading the entire history of bitcoin prices can be slow/unreliable.

If Spotbit is to have a database of prices, it should be as easy as possible to maintain the code and ensure the integrity of the data.

To that end:
- Teach Spotbit to import files from sources that provide `csv` files of all recorded price history:
- [ ] Yahoo Finance
- [ ] Coindesk
- [ ] blockchain.info
- [ ] Google's BigQuery
- [ ] Store these prices in sqlite for easy querying.
- [ ] Add an option that allows Spotbit to use it's saved data offline instead of querying exchanges.