Allow setting of abitrary php.ini configs via directives attribute

The opscode PHP cookbook allows setting of any config variable in `php.ini` via a `directives` attribute, it would be useful to have it in here as well. It is a bit more lazy than creating an attribute for every php.ini setting...

For example when configuring a Symfony app server with the php cookbook I use:

```ruby
default_attributes "php" => {
  "directives" => {
    "date.timezone" => "Europe/London",
    "short_open_tag" => "Off",
    "magic_quotes_gpc" => "Off",
    "register_globals" => "Off",
    "session.autostart" => "Off"
  }
}
```