Store Init In XDG App Dir (#363)

* Store configurations in the correct XDG App Dir

Read the init file from the XDG config app dir using the app_dirs crate,
and set the HISTORY_FILE to be located in the XDG data app dir.
The init file is now named initrc instead of .ionrc.

* Enable history file by default

Set HISTORY_FILE_ENABLED to "1" so that ion writes history by default.