add a changelog file, prepping for v0.1.1

The v0.1.0 notes are copied from the GitHub release.

add a -version flag to gofumpt

This allows for versioned go get/install commands to bundle the module
version in the binary, then the version flag can print it as-is.

Prebuilt binaries will also include it via ldflags.

Not for gofumports, since its days are counted.

For mvdan#99.