Optional Assignment Operator

It can be useful to optionally-define a variable if it has not been set, as is often seen in Make files.

let FOO ?= BAR
# FOO is now BAR
let FOO ?= BAZZ
# FOO is still BAR

Should make the following possible:

$ env OPT=FOO ./script.ion
OPT is FOO
$ ./script.ion
OPT is BAR