add iexcloud provider

[cshanes]

Used gosuto's IEXCloud example from the Signals example scripts repo to add to OpenSignals. Here's an example snippet for how to use it:

from pathlib import Path
import pandas as pd
import numerapi
from opensignals.data.iexcloud import IEXCloud
from opensignals.features import DividendsAPY

db_dir = Path('db')

iexcloud = IEXCloud()
iexcloud.download_data(db_dir, region='US')

features_generators = [
    DividendsAPY()
]

train, test, live, feature_names = iexcloud.get_data(db_dir, features_generators=features_generators)

Let me know if there's additional documentation you'd like me to add

[jaapterwoerds]

Nice work @cshanes.

[jaapterwoerds]

I would rather could override / decorate the get_tickers method to do this mapping. In that way the implementation specific knowledge stays within the IEXCloud provider. The region parameter could be a constructor parameter of IEXCloud provider.

[jaapterwoerds]

From a quick look at the iexfinance github readme, it seems like you could pass the token to the library functions and also instance when instantiating the Stock object.