Provide a way for attribution to include a source date

[stefanb]

Some source licenses require attribution to include a date of the source data.
Eg Slovenia: http://www.e-prostor.gov.si/fileadmin/struktura/ANG/General_terms.pdf

The data user shall indicate the data source in every publication of data or products. Such indications
shall include the following data: "Surveying and Mapping Authority of the Republic of Slovenia, the
type of data and the time to which the data refer and/or the date of the database."

To keep it simple for Slovenia i have reduced the "date" component of attribution to a year-only precision in /sources/si/countrywide.json, but updates have to be done via pull requests and update from 2017 to 2018 is still waiting in openaddresses/openaddresses#3470 (see sources/si/countrywide.json)

Ideally there would be a way to extract some metadata from the source and allow that changes to attribution to be automated.

The daily prepared souce zip file for slovenia contains a .txt file with exact date, but that could easily be changed to provide the date in some other way, eg:

• a json with required attribution (a sort of updated local countrywide.json distributed with the zip file itself)
• include attribution with every row in the .csv (machine would collect them, and list all the unique attributions in the final attribution)
• include date with every row in the .csv (machine would collect them, and list all the unique attributions in the final metadata, or provide a range)
• check the timestamp of csv within the zip file

A simple, but not exact solution would be to allow some placeholder replacement within attribution text (eg to replace {yyyy-mm-dd} with current date and {yyyy} with the current year).

(ticked moved over from openaddresses/openaddresses#3961)