Using the FISH_HISTFILE variable will let people customise the session
to use for the history file. The resulting history file is:
`$XDG_DATA_HOME/fish/name_history`
Where `name` is the name of the session. The default value is `fish`
which results in the current history file.
If it's set to an empty string, the history will not be stored to a
file.
Fixes#102
