[WIP] Feature/csv to sql natively

When finished, this PR should address #352. New `into` function calls out to to native COPY function of various DBs.  Has optional `if_exists` argument (similar to pandas) to handle three cases: replace, append, and fail

```python
into(sql,csv, if_exists="replace")
```


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What if the dialect is neither `postgresql` nor `sqlite` or what if the dialect is `sqlite` and the platform is windows?  We should probably default to `sql.extend(csv)` in many cases.