Generates a context module with functions to work with a tree, along with
its schema and migration (see mix ancestry_ex.gen.schema).
$ mix ancestry_ex.gen.context Catalog Category categories name:string
Generates MyApp.Catalog with list_categories/0, list_root_categories/0,
list_category_children/1, list_category_ancestors/1, category_tree/0,
category_subtree/1, get_category!/1, create_category/1,
update_category/2, move_category/2, delete_category/1 and
change_category/2.
If the context module already exists, the functions are appended to it.
Function names come from the plural name (categories), the table name
can be changed with --table.
Accepts the same options as mix ancestry_ex.gen.schema.