Add JSON/YAML marshalling/unmarshalling

[kke]

This allows k0sctl to do something like:

type K0s struct {
  version *version.Version
}

and have it automatically convert the string to a proper version object, making comparisons easier (with #4 specially) and reduce doing multiple conversions.

[kke]

The YAML tests are a bit weird so that the dependency to a yaml package could be dropped, but as seen before the second commit, the tests passed just fine using the gopkg.in/yaml.v2 dependency and removing the dep only required changes to the tests, otherwise the code was untouched.