Extensibility/Flexibility

Currently, there are many SQL statements that a user might want to use, that are not expressible in Sclojure (e.g. CTEs in Postgres https://www.postgresql.org/docs/9.1/queries-with.html).
Of course the library cannot support everything, esp. if it is outside of relational algebra, but when writing an application and one has already written many parts of the queries in sqlosure, then if one reaches a point where a (small) part needs something special, then nothing of the previous parts can be reused.
Therefore, I think that an extension mechanism would be very helpful.

As quick scratch, a way to embed a raw sql query could be added for that:

```
(embed-sql sql-string columns) ;; + query args maybe
```

What might be equally flexible would be a way to add new Sqlosure expressions as a user, by having an api to define their translation to SQL.