Feature: add intra-day support for tiingo to getSymbols

[ethanbsmith]

getSymbols.tiingo currently only supports daily-monthly data sets. However tiingo does have an API sitting on top of IEX data that support intra-day granularity: https://api.tiingo.com/docs/iex/realtime#historicalPriceData

It should be fairly straight forward to add this support to getSymbols.tiingo, using an interval parameter (like alphavantage). However, since this intra-day API sits on top of IEX, it does not support the full set symbols that the EOD API does. Another option would be to add a new getSymbols.tiingo.iex. My preference would be to just extend the existing function. Any thoughts before I start on a PR for this?

[ethanbsmith]

Will revisit this when tiingo release v2 of their API in 2019