Implement caching

[jemus42]

First up: The way deployment to shinyapps.io works is that everything the application writes to disk does not survive consecutive deployments, i.e. the vm the app runs in gets destroyed and a new one is created. Therefore caching is even more temporary that one would assume.

However, I think we should create a cache dir, and every series dataset that gets pulled should be saved in the most efficient way (either via save or saveRDS, which would probably be more flexible) to said directory, and on startup the app should be able to compare the query (or download the search result and then compare id_tvdb or something) and if the dataset is present in cache, read the data from there.

We could also download some popular series' data and permanently commit it to the repository, as example data. This would probably be helpful for debugging purposes as well.

[jemus42]

Done