Use sql comments as output by `diesel print-schema`, if present

How should this be supported? - I'm thinking:
```sql
CREATE TABLE "profiles" (
    alias             VARCHAR(50) PRIMARY KEY    -- user-modifiable alias
);
```

Or maybe:
```sql
COMMENT ON COLUMN profiles.alias IS 'user-modifiable alias';
```

Or something else? - What do you think?

Here's the current code FWIW: https://github.com/Wulf/dsync/blob/2797966/src/code.rs#L360