Configuration function to get "shifted" IDs from config file.

[immibis]

Currently, Configuration.getItem gets an "unshifted" item ID from the config file, which is the actual ID minus 256.

There should be a similar function that gets a shifted/actual item ID from the config file, because storing the wrong numbers in the config file only confuses people.

[LexManos]

Its been a pretty standard thing to store the pre-shifted id in the config. Also adding a second function to store a shifted id would just be confusing even more.