AddDbContext method for DI

I think being able to pass DbContextOptions into ctor is super useful, especially when your app uses DI but for testing you just want to create the options and pass them in. The ```IOptionsAccessor``` thing is difficult to understand, so I don't think we should expose those from the ctors.

Seems like there is a lot of questions about how all this stuff actually works, so we should discuss in person rather than trying to solve it here.