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'm wondering wether we want to treat those options explicitly or if it would be better to just provide a raw SQL extension point. MySQL for example has `EXPLAIN EXTENDED`, which could also benefit from that. I'm think about something along the lines of:

```ruby
explain modifier: "ANALYZE"
```