[Enhancement] tngoLast added to IEX Intraday Quotes Acceptable Fields

* Tiingo Python version: 0.13
* Python version: 3.8.5
* Operating System: Ubuntu 18.04 LTS, Linux

### Description

Tiingo has a useful field for Intraday quotes called tngoLast which is either A) the last traded price or B) a Tiingo calculated field which, I assume, is based on current Bid/Ask if the update to the quote is considered 'major', e.g. the last traded price is not between the current bid/ask.

### What I Did
N/A
```
We would need to set another, optional, parameter and include the field in the acceptable list within 'metric_name'
```
This is another I may spearhead myself. This is just a placeholder in case somebody beats me to the punchline.