Treat missing sections as empty, not as error
Treat missing sections as empty, not as error

This allows you to execute cargo list for a crate that does not have any
dependencies. A downside of this approach is that you don't get an error
anymore if you misspell the section name.

Sample `Cargo.toml` that it rejected before:

    [package]
    name = "name"
    version = "0.0.1"
    authors = ["Author Name <author@example.com>"]

killercup/cargo-edit,dandroid/cargo-edit