Feature request - get_last_price(tickers)

* Tiingo Python version: 0.13.0
* Python version: 3.8
* Operating System: OSX

### Description

The most common use case is to get snapshot price of a portfolio of tickers. It would be nice to have a simple api that takes a number of tickers and returns the last price. It sure can be implemented using existing apis like get_dataframe, but that is going to be inefficient in many ways for following reasons.

1. You have to guess the correct start date. Creates a problem when 'now' is a holiday or weekend
2. You end up discarding all the data except the last row for each ticker - reduces network traffic
3. We most likely only need the adjClose and possibly the time/date of that price - reduces network traffic

### What I Did