Cache response bodies from KEGG

[erikhuck]

There should be an option to cache what is pulled from KEGG REST and then access the cache instead of sending redundant html requests. Likely this cache would be implemented in a .kegg_pull_cache.zip file in the current working directory.The url path itself could be used for structuring the filepath in the cache. There should be options to turn on and off the caching. Also, there should be an option to use a specific cache file. Caching could greatly speed up certain operations while also reducing needless redundant network traffic.