1.0.0-beta.1

It's happening, folks! miette is now working towards stability and is now in
beta! We'll keep it like this for a little while until a few more people have
tried it out and given feedback. New features may still be added, and breaking
changes may still happen, but miette is now considered "good enough to use",
and breaking changes are expected to be more rare.

Oh, and as part of this release, the docs were overhauled, particularly the
README, so you might want to take a gander at them!

Breaking Changes

• printer: rename default printer and consistify some naming conventions with printing (aafa4a3d, breaks #)
• into_diagnostic: .into_diagnostic() is now generic across any impl fmt::Display instead of expecting a dyn (c1da4a0d)

Features

• error: diagnostic-ify MietteError (e980b723)

Bug Fixes

• derive: #[diagnosic(severity)] works for named and unnamed variants/structs now (adf0bc93)
• protocol: oops, missed a spot after a rename (5c077d30)