Add refresh cache option

Maybe for functions add `refresh_cache = FALSE` to refresh just the requested dataset.

Then a simple `refresh_cache()` function to clear the entire cache? Could specify to just unlink part of the cache (like a year or geography, zip files, or variables).

```
cache_dir <- user_cache_dir("tidychas")

if (cache = "all") {
  unlink(cache_dir)
}

...
```