Iceland - additional function in API to get a specific date

[Stalfur]

Hello, we have previously opened up an API for Iceland AQ data in #552

There you could only fetch the last 24 hours, we have now added a new function there, getDate which loads the data for the selected date.

Example: https://api.ust.is/aq/a/getDate/date/2018-01-01

This is using the same structure as getLatest, the only difference is the input parameter (none for getLatest, valid date for getDate)