Add an option to call gofmt or alternative script just after writing the generated file

[moul]

This is a bonus, is as we can easily do it from a command just after calling protoc,

Hiwever it allows to ensure every generated go files are formatted and checked for syntax error even if the script fails on the nth file in a batch

[vtolstov]

this is really simple, like this: https://gist.github.com/vtolstov/c9327a3ef23313891bb050163b3df8ea