Enhancement: Provide option to modify cache folder for entity linker knowledge base downloads

https://github.com/allenai/scispacy/blob/2290a80cfe0948e48d8ecfbd60064019d57a6874/scispacy/file_cache.py#L16

For Google Colab users, the `Path.home()` location is `/root/`, which is deleted when the runtime is cleared. As runtimes are cleared fairly often, this means re-downloading the KBs. Perhaps there is a way to alter `Path.home` from `pathlib`? Another option is to allow the user to enter a cache folder, which Colab users could set to their Google Drive (fwiw just a regular folder as seen by python within Colab), thus making the download permanent.