Comment support

[rickyvetter]

It would be nice to support comments inside the GraphQL parser. Currently something like this doesn't parse at all:

module UserQuery = [%graphql
  {|
    query {
      thing {
        name # comment
      }
    }
  |}
];

It also gives a pretty vague error - "Unknown character" on the line after the #.

[Neitsch]

This can be closed #49 .