Pass interaction to default procs

Fixes #217. Example usage:

``` rb
class Example < ActiveInteraction::Base
  string :a
  string :b,
    default: 'b'
  string :c,
    default: -> { 'c' }
  string :d,
    default: -> this { [this.a, this.b, this.c].join(' ') }

  def execute
    inputs
  end
end

p Example.new.inputs
# {:a=>"a", :b=>"b", :c=>"c", :d=>"b c"}
p Example.run!(a: 'a')
# {:a=>"a", :b=>"b", :c=>"c", :d=>"a b c"}
```