FORCE QUOTE flag can't be specified in flags

Example query that I'd like to generate:

```sql
COPY (
select id
from customers
limit 1
)
TO STDOUT
WITH CSV HEADER QUOTE AS '"'
FORCE QUOTE id;
```

However, due to the logic in `format_flag()` it's not possible to pass `force_quote="id"` or `force_quote=["id"]` or `force_quote=("id",)` when calling `copy_to()` as it calls `repr()` in each case on those values.

I'm happy to create a pull request if this project is still being maintained?

Perhaps a solution could be to whitelist certain flag names and treat them differently when formatting?