Don not ascend above --ceiling when looking for justfile

[casey]

Add a --ceiling option, which can also be passed as JUST_CEILING. When searching for a Justfile to run, or for a project root in which to initialize a justfile, don't ascend above the ceiling.

For example:

$ pwd
/foo/bar/baz
$ echo foo: > /foo/justfile
$ just --ceiling /foo/bar
error: No justfile found