Fix build via #32

Support tool directive via #31

Also replaced pkg/errors with std lib

This is a patch release that fixes the following issue: #24

This release fixes a TODO that updates go.mod files when passing the --mod-name flag to update client dependencies.
Ref: 7d1d91d

Ignore go build constraints thanks to the contribution by @pascal-hofmann

Support gopkg.in upgrades/downgrades.

Thanks to @maxbrunet for the contribution

Fixes #16

Thanks to @horgh and @kk-zu for the issue and the pr.

Thanks to @mchill for the PR

Fixes a go get issue now that urfave/cli has adopted Go modules.

#12 @sauerbraten

Fixes #10