Create a semver tag for 0.3.0 with a GH release

[displague]

Thanks for updating powerline-rust!

I'd love to learn what changes have taken place by glancing at https://github.com/cirho/powerline-rust/releases

[cirho]

Sorry for the delay,
in few days there will be a new 0.4 version released and with it I'm planning on starting a changelog.