Let Writer preserve format

[soutaro]

I found some RBS files has manually formatted definitions like:

type expr = IntExpr
          | StringExpr
          | FuncallExpr

def foo: (some, params,
          many: bool,
          keyword: String,
          ?parameters: bool) -> void

And I don't want to destroy the good format.

I add an option #preserve? to Writer to re-use the source text from which the ast is constructed.
The option works only for type decl and method definitions.