[dev-context] piping to ConTeXt

Hello Hans, Asymptote developers wonder if there's a way to pipe to ConTeXt. I didn't start inspecting anything as I suspect that it's quite possible that nobody has ever tried that so far. The trick that John Bowman decided to use was to "\input null" (an empty tex file "null.tex"), but I already had problems because I (forgot that I) didn't have that file. Mojca