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.
what about:

```ruby
sql = "EXPLAIN"
sql << " (#{options[:modifier]})" if options.key? :modifier
sql << to_sql(arel, binds)
```