This changes the default value of `dotenv-load` from `true` to `false`. This
is a backwards incompatible change, and will require a minor version bump.
This commit adds a `Loader` type, which can be used to load multiple
source strings. This was done to support the work on modules, but
coincidentally enabled consolidating errors, since now `Config::run`
can take a `&Loader`, and in the event of an error, return and `Error`
that borrows from loaded strings. Multiple error types have been
consolidated, and a bunch of ad-hoc error printing was removed.
