A CLI command to create a directory tree for a new out of tree applet

[whitequark]

Once we enable the general use of out of tree applets (which should be possible soon, as long as we figure out API versioning and make it exceedingly clear that the current API is unstable and will be regularly broken without warning), we should add a command for creating a new out-of-tree applet (with the right dependencies, and a template filled in with the user's provided name) that also prints (or includes in the applet source tree) instructions for its use.