Rewrite Fathead tutorial

I had to fill in some blanks by referring to the plugins in the repo, so there may be some missing details that I didn't know about.

One thing that still confuses me: What is the relationship between the name of the Perl module and the name of the directory? In this example, the module is `HelloWorld` but the directory is `hello_world`. Is there any rule to it?

Also, I think the parse.py in here is incorrect—it doesn't have the right number of fields. I would have used the one from the actual Fathead repo, but I didn't know if there was some reason @moollaza decided to reimplement it, so I just left it alone for now.