Add .actions method for index as table

I've always wanted to be able to easily extend the default actions. This adds an `actions` method when using index as table.

Some examples:

```ruby
# Works just like before
default_actions

# Same as above
actions

# Append some actions onto the end of the default actions
actions do |resource|
  auto_link resource
end

# Custom actions without the defaults.
actions :defaults => false do |resource|
  auto_link resource
end
```

Would love some feedback/suggestions.