Caching is slow

[CristianCantoro]

The caching strategy is dumb.

Requests are cached simply continuously reading and writing the JSON cache file. As the cache gets bigger this process becomes the bottleneck of the script. It is enough to have a cache in the order of tens of MB to slow down the script consistently.

The current caching mechanism is good only for development purposes to avoid making requests to the Wikisource API while developing. For this reason caching is optional and must be explictly activated with the --enable-caching flag.