Include all fields returned by Yahoo in getOptionChain

[joshuaulrich]

Only some of the fields that Yahoo Finance returns for options are returned by getOptionChain(). We could return all of the fields.

[semiographics]

I updated getOptionChain() to include the five Yahoo fields that were not already in the process.

Some notes:

"contractSymbol" is useful as a primary key.
I converted "expiration" to POSIXtc in order to facilitate analyzing theta decay.

Yeah?

[joshuaulrich]

Thanks for taking the initiative to make a PR for this!