Automate code formatting, tool installation in make

[juliaogris]

We should probably have a make target for formatting go code (currently the contributors have their editors set up accordingly). Along with it we should have make targets checking that the code is formatted, e.g.:

 make fmt
 make check-fmt

According to a PR conversation, we might want to use an extension to gofumports for code formatting. We might want to consider moving this extension into a separate script files directory, maybe even repository.

We might also want to consider adding:

 make get-tools

To download gofumports and|or scripts.

[juliaogris]

We might also want to work out linter version derivation in a separate script:
https://github.com/foxygoat/jsonnext/pull/25/files#r495622807