[feature] Permit multiple metrics to be fetched from multiple tickers as 1 library call

## Lineage

1. The library was specified that multiple tickers only support single metrics in the docstring but not in error messages, leading to a confusing error message in [here](https://github.com/hydrosquall/tiingo-python/pull/258).
2. The error message was clarified in #263.
3. Getting multiple metrics back in 1 API call from multiple tickers was requested as a new feature [here](https://github.com/hydrosquall/tiingo-python/pull/263#issuecomment-487978305)

In the meantime, users have 2 alternatives

- Use something like the code snippet proposed [here](https://github.com/hydrosquall/tiingo-python/pull/258#issuecomment-487808469)
- Share a bit about how the data structure you'd expected for a multi-metric dataframe with multiple tickers might look like (would wide/long tables work for your usecase, or would you prefer a multi-indexed dataframe)?