Adding market_identifier_code to security

[ghbutton]

I was wondering what you think about adding market_identifier_code to the %Plaid.Investments.Security{} struct.

Documentation: https://plaid.com/docs/api/products/investments/#investments-holdings-get-response-securities-market-identifier-code

[ghbutton]

Ohh and perhaps we can add sectory and industry as well?

[wfgilman]

Happy to merge a PR to support these fields

[ghbutton]

Cool I can make that, thanks!