Allow erasing multiple variables, abbrs, etc.

[IlanCosman]

The functions --erase command accepts multiple arguments, erasing all of them. set and abbr should do the same. I've not infrequently found myself using the

for var in $list_of_vars
    set -e $var
end

idiom. set -e $list_of_vars would be much more convenient. Similarly with abbr, though in truth I've never needed to erase multiple abbrs at once.

Either the functions --erase command should not accept multiple arguments, or set --erase and abbr --erase should (obviously I believe the latter).

[ridiculousfish]

Seems reasonable!