Store keras data in XDG_CACHE_HOME

Updates #17
 - Changes the location of keras data from `XDG_CONFIG_HOME` to
   `XDG_CACHE_HOME`
 - This is done by modifying `KERAS_HOME` in `.zshenv`
 - The reasoning behind this change is that despite the fact that one
   can specify the download path for keras datasets, it's always
   relative to `$KERAS_HOME/datasets`. So storing such a large cache in
   `XDG_CONFIG_HOME` would not be optimal.

To apply this change one can simply move the directory.

.config/zsh/.zshenv

@@ -164,8 +164,9 @@ export TFDS_DATA_DIR=${XDG_CACHE_HOME}/tensorflow_datasets
 
 # Keras
 #  - https://github.com/tensorflow/tensorflow/issues/38831
-#  - Since the only file is `keras.json` config, `XDG_CONFIG_HOME` is used here
-export KERAS_HOME=${XDG_CONFIG_HOME}/keras
+#  - Since `KERAS_HOME` contains config `keras.json` which is re-created when
+#    missing and cached datasets, `XDG_CACHE_HOME` is probably the best option
+export KERAS_HOME=${XDG_CACHE_HOME}/keras
 
 # Dungeon Crawl Stone Soup
 #  - Note: the trailing slash is required