extend getQuote to support tiingo

[ethanbsmith]

I'm going to work up a PR for getQuote functionality for tiingo. I have a simple version working already, but there are a few design decisions i'd like some feedback on first:

tiingo returns 17 columns that are documented at https://api.tiingo.com/docs/iex/realtime#realtimeData . This raises the following questions:

1. Should column names like LastsaleTimeStamp be changed to Trade Time and first letter of column names be upper cased to make is more of a direct substitute for the yahoo and av versions? my vote on this would be yes to case changes and no on column names changes, but to add a Trade Time that is a copy of LastsaleTimeStamp
2. tiingo returns 17 columns should all columns be passed through to the caller, or should it just default to OHLCV and the what functionality from yahoo version be supported? my vote if to return all 17 as its not overly burdensome and has no impact on performance. I will reorder the columns to put OHLCV first.

[ethanbsmith]

initial version in https://github.com/ethanbsmith/quantmod/blob/247_getquote_tiingo/R/getQuote.R