Add the ability to alter the Z datafile path

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Prefix command, when executing non-builtin commands

This way custom functions/aliases, defined in the config.fish, don't mess up the plugin.
Merge remote-tracking branch 'kergoth-fork/datafile'

Conflicts:
	z.fish
Merge remote-tracking branch 'Mic92-fork/patch-1'

Conflicts:
	z.fish

A better name for this branch would be ‘safe-commands’
Remove obsolete instruction to modify fish_prompt