Fix handling of directories that look like flags in fish

[schuelermine]

When installed in Fish, z -C or z -s or similar flags unrecognized by Fish’s string replace error out with:

string replace: -s: unknown option

- (line 1): 
in command substitution
	called on line 56 of file -
in function '__zoxide_z' with arguments '-s'
	called on line 1 of file -
in function 'z' with arguments '-s'

(Type 'help string' for related documentation)

This addresses that by adding a -- separator in the string replace call, which suppresses interpretation of flags.