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.

[tconbeer]

I like this idea.

I think we'd support this by installing a markdown parser as an extra (e.g., mistletoe or marko), iterating through the code blocks in the AST, and replacing their contents.

Right now we ignore .md files, but that can be changed via config.

PRs very welcome!

[michael-the1]

Looks doable! I'm able to spend some time on this so I'll try to get a PR up sometime in the next two weeks.