Introduce prepending and appending to a literal define list (+=, =+)

[mkruselj]

Enables fun stuff like:

define FOO := name
define FOO += surname
define FOO =+ how, about, them, apples

on init
    literate_macro(declare #l#) on HELLO
    literate_macro(message(#l#)) on HELLO
end on

which results in:

on init
    declare $how
    declare $about
    declare $them
    declare $apples
    declare $name
    declare $surname
    message($how)
    message($about)
    message($them)
    message($apples)
    message($name)
    message($surname)
end on

Wahey!