Remove all external dependencies

github.com/spf13/pflag isn't netting us as much as I had hoped it would,
and it's a pretty decently-sized dependency; ultimately not worth the
effort.

github.com/pkg/errors is gone too, now that the standard library has
everything we need.

Merge branch 'pflag'

go.mod: Pull in dependencies.

Add go.{mod,sum}.

readme: add link to godoc