Description:
    Creates a DB migration to generate the schema for a table which is used to
    store the entries of an eav_hash.

Example:
    If you're not setting a custom table name, i.e., your model looks like this:

        class ModelName < ActiveRecord::Base
            eav_hash_for :hash_name
        end

    Then simply run this migration like so:

        rails generate eav_migration ModelName hash_name

    However, if you're a wizard and have specified a custom table name like this:

        class ModelName < ActiveRecord::Base
            eav_hash_for :hash_name, :table_name => :custom_table
        end

    Then run this generator like this:

        rails generate eav_migration ModelName hash_name custom_table

    Easy as pie!