Option to add additional comments in the generated ORM files.

[nohhyun]

I am wondering if people would be interested in having an option to add a specific string to the top of the generated file.
I am thinking something like this can be added for the sqlboiler.toml.

output   = "models"             
pkgname  = "models"             
wipe     = true                
no-tests = true
add-enum-types = true  
additional-top-comment = "@generated"

Which will generate files with

// Code generated by SQLBoiler (https://github.com/volatiletech/sqlboiler). DO NOT EDIT.
// This file is meant to be re-generated in place and/or deleted at any time.
// @generated

at the top.
I can generate a quick pull request if this seems acceptable.

[stephenafamo]

PR welcome 👍🏾