Adding option strategies support

[4rumprom]

Extending functionalities to enable option trading strategies supported by Schwabs.

Adding to the Schwab class:

• API call function to pull option chain quotes for an underlying symbol. This optionally includes option greeks.
• Specific option trading function to create 2 to 4 legged strategies such as spreads, butterflies, condors etc.

Also included an example file that pulls the option chains, helps normalize the json response into a pandas dataframe and creates a dry run on an option straddle strategy.

[4rumprom]

I made the requested changes.

I like the idea of class-wide enumerators for the different option strategies or order types. I'm not totally sure on the best approach. I'd like to get the current pull request approved as is for now. It's probably only one of more pull requests to come.

With options, I have a big question mark around handling option assignments before expiration. I saw that account positions have child positions. I wonder if that's where those will be referenced at. I hate that Schwab doesn't come with a paper trading account to figure this out.