Support multi-line SQL comments

[soroushj]

Multi-line SQL comments broke the code generator. This PR fixes the issue.

The case: Consider this SQL schema:

CREATE TABLE authors (
  id   bigserial PRIMARY KEY,
  name text      NOT NULL
);

COMMENT ON TABLE authors IS
'Keeps authors.
Contains id and name.';

Previous behavior: The code generator (sqlc generate) exited with an error:

...
// Keeps authors.
Contains id and name.
type Author struct {
  ID int64 
  Name string 
}


# package db
error generating code: source error: 17:1: expected declaration, found Contains

The reason is obvious: The code generator adds // only at the beginning of the comment string. As a result, only the first line of the comment string is commented out.

New behavior: Code generator adds // at the beginning of every line of the comment string. For the aforementioned schema, the models.go file will be generated as:

...
// Keeps authors.
// Contains id and name.
type Author struct {
	ID   int64
	Name string
}

The fix:

• Implemented for both Go and Kotlin code generators.
• Works correctly for both Unix and Windows newlines (LF and CRLF).
• The changed files are go fmt'd using go1.13.4 linux/amd64.

[kyleconroy]

Looks great! Thanks for code