Add a flag to change which type to iterate over in order to create files

[pmoroney]

I needed to generate a file per message as well as a single file with access to all of the AST.
So I added a type flag that you can pass the following options:
message: Generate a file per message definition, allows you to create files like {{.Message.Name}}.go.tmpl
service: Generate a file per service definition. This was the previous behavior and is now the default.
file: Generate a file per .proto file. This allows you to generate per file templates if those proto files have multiple service definitions.
none: Generates a single file. This gives you access to .Files which is a slice of all the .File descriptors.

[moul]

Hi @pmoroney,

Can you add an example in the examples directory ?
It will be easier to discover the flag features and that example can be used as kind of integration test

Thank you 😊

[pmoroney]

Thanks for your patience, I will get to this as soon as I can :)