proto comments are not transferred in the generated files

[anmolitor]

.proto files can be documented by leaving comments in the file, which are picked up by protoc and exposed
as SourceCodeInfo. We could use that info to move the comments to fitting places in the generated elm code.

Example:

// A simple message
message Simple {
  // a field containing a string
  string field = 1;
}

{- |  A simple message -}
type alias Simple = {
  -- a field containing a string
  field : String
}

[anmolitor]

Elm Syntax DSL does not support comments outside of the root scope. It's not easy to add, since it aims to be compatible with elm-format, plus elm-syntax does not support it either, so it is an upstream issue. Therefore elm codegen has the same limitation.

Issue is here: mdgriffith/elm-codegen#39

This is not a blocker though, I've decided to implement this case a little differently.
For the example above, the comment that gets generated looks like this:

{- | A simple message

## Fields
### field
A field containing a string
-}

This is now live in version 3.1.0 and above :)