Support IF NOT EXISTS for table creation

Currently, I have a bunch of code that looks like this:

```python
subjects = db["subjects"] if db["subjects"].exists() else db["subjects"].create({
    ...
})
```
It would be neat if sqlite-utils could simplify that by supporting `CREATE TABLE IF NOT EXISTS`, so that I'd be able to write, e.g.

```python
subjects = db["subjects"].create({...}, if_not_exists=True)
```