Add support for line item options

This adds a foreign key from Transfer to LineItemOptions (as suggested by Jordan in #1119) and updates the accounting controller and StudentExtraCosts module to make use of it.  You can test this by creating new line item types for a program; you can add options (each with a description and optionally a new cost value) from the admin page for the line item type.  You should see each of these options on the extra costs page in student reg, and it should remember your choices when you save and come back later.

Addresses #1119 and #1120.  However, it could be argued that more front-end work is desirable for #1120.  Also, we don't support a combination of multiple LineItemOptions choices and variable quantity (in the StudentExtraCosts module; the accounting controller should be able to handle it).

Can we name this ```option``` (singular)? I know the model class name is plural. But the singular makes more sense as an attribute name.

Maybe at some point (not in this pull request) we can singularize all of the model class names that are plural (ClassCategories is another example). Along with that, we can either rename the associated tables in migrations, or just keep the original table names with the appropriate Django meta value.