Add flag to prepend comment to file

[metalmatze]

I'd like to add a comment like // Code generated by gojson. DO NOT EDIT. to my generated file. I don't want to do this manually. So I propose adding -comment flag which would just prepend that line to the file.

[ChimeraCoder]

Yeah, this is a good idea. I actually can't think of a reason not to include this in all files (many other code generation tools in Go already do), so we may not even need to add a -comment flag to do so.