Cartfile.private

Adds support for a `Cartfile.private` file, which can be used to record a project’s private dependencies (those that should not be considered by parent projects).

Resolves #125.

**To do:**

 - [x] Update documentation
I definitely prefer the “dependent” terminology, but I think it's easy for readers (especially those not very fluent in English) to get confused between “dependent” and “dependency.” Hopefully the parenthetical just clarifies what it means.