Add a script to run 'go vet'

@filbranden 
`go vet` seems to be able to take multiple arguments at once, so maybe this would be simpler (to replace about all of the script):

```shell
if [[ "${targets[@]+set}" != "set" ]]; then
  targets=("...")
fi
go vet "${goflags[@]:0}" "${targets[@]/#/./}"
```

So targets will default to a single `"..."` and the last `/#/./` will add a `./` prefix to all targets before calling `go vet`.

If go vet fails the script will propagate its exit status already.

The `grep -v` is not there. Feel free to re-add it if you really like it, in which case see my other comments...

This basically replaces line 40 forwards.

Done! :-)