Improve parsing comments in protofiles to retain formatting when emitting ocaml doc

[andersfugmann]

Documentation in googles .proto files seems to be a mix of markdown and indentation to inform formatting.

Lists are on the form:

  - item a is described here and 
    this is a continuation of list item
  - Item b is explained here

Code blocks seems just to be indented lines:

There is an example
   message A { 
     uint32 x = 1;
   }
In the example we see that ...

Regular paragraphs are non indented. Some words contains annotations:

• Strings enclosed in backticks are verbatim
• Urls are on the form text. Text may be formattes aswell.
• String enclosed in * should be marked as bold

List items contains paragraph and may also contain annotations