Whitespace pedantry
Add simple data model for menus.
Schema update
Load menus in invitation show.

There will be better ways of doing this if people actually want to
take this engine and run with it, but i’m not going to pre-empt that.
Decided that loading an invitation is an edit, not a show.
Basic method and spec for updating an invitation (RSVP)
Using !-ed update was causing problems, remove it

Initially was using update! because couldn’t think of a reason
why this would fail, so wanted an exception, but controller and
spec became complicated trying to account for various different
exceptions that could be raised, so just went for plain old
update instead.
Link guests to dishes via menu choices
Added todos to readme