options for Relation.explain

Also support for postgre's ANALYZE & BUFFERS options

It adds ability to pass custom options to `Relation.analyze`.
I've implemented support for postgre's options. Mysql's `EXPLAIN EXTENDED` can be implemented later.
I doubt anyone will be feeding this debugging function untrusted input but probably better safe than sorry. A whitelist shouldn't be terribly hard to implement. The available options are:

```
 ANALYZE [ boolean ]
    VERBOSE [ boolean ]
    COSTS [ boolean ]
    BUFFERS [ boolean ]
    TIMING [ boolean ]
    FORMAT { TEXT | XML | JSON | YAML }
```