Determine how to store API keys for exchanges

[pegahcarter]

I think we have two choices:

• one file per exchange, naming the file the exchange name
• one file that contains the key/secret for every exchange

And with those two choices:

• Is there a choice that is more user friendly?
• Is there a choice that's easier to add exchanges?
• Will they be premade, so the user only inputs the key/secret?