Provide a Shellable[Option[T: Shellable]]

[lihaoyi]

This should work

@ val x: Option[String] = Some("cat") 
x: Option[String] = Some(value = "cat")

@ os.proc(x, ".editorconfig").call() 
cmd1.sc:1: type mismatch;
 found   : Option[String]
 required: os.Shellable
val res1 = os.proc(x, ".editorconfig").call()
                   ^
Compilation Failed

[lefou]

I had thought, we already support it with IterableShellable, but turns out, Option isn't an Iterable but an IterableOnce.

[lihaoyi]

Looks like this is fixed in #171