Data caching + reading local files

[pitkant]

It might be a good idea to implement data caching in vipunen, similar to rOpenGov/sotkanet#14 and eurostat's implementation (see set_eurostat_cache, get_eurostat rows related to reading the cache). Additionally, implementing this might make it easier to implement functionality related to reading files locally, i.e. if you have a downloaded .csv file that contains the same data as the API does.