Support for formatting SQL code blocks in Markdown

[michael-the1]

It would be really nice if sqlfmt would support formatting SQL code blocks in Markdown files. In the context of dbt, this would also be very beneficial. We put a lot of example queries in our documentation and it would be amazing if they followed the same style.

Before

# Some title

Some text

```sql
SELECT * FROM some.table
```

After

# Some title

Some text

```sql
select * from some.table
```

In terms of requirements, I think limiting to sql blocks within .md files would be enough.