Ability to pass block to select form builder helper

In case when there is some complicated html inside of <select> tag the default rails helpers won't help and you need to generate html manually.

This patch allows to pass a block to rails form builder helper:

``` haml
    = select(report, "campaign_ids") do
    -# OR
    = f.select(:campaign_ids) do
      - available_campaigns.each do |c|
        %option{:data => {:tags => c.tags.to_json}, :value => c.id}= c.name
```