Use a semver compatible numbering

[TeXitoi]

By semver implementation in cargo, all version 0.0.x are incompatible, technically generating a breaking change to upgrade flat_map. But almost all changes are just adding features.

So I propose the next release to be numbererd as 0.1.0. Like that, the version after can be 0.1.1, and upgrading to 0.1.1 will not be a breaking change for the dependent crates (and will be done automatically by a cargo update in a crate using a crate depending on flat_map).