Allow `its` keyword to take parameters

Created by: JoshCheek

In pretty much every project I find myself wanting to describe objects through methods that take parameters like this:

class Person
  def initialize(first_name, last_name)
    @first_name, @last_name = first_name, last_name
  end

  def name(format)
    format.gsub('%f', @first_name).gsub('%l', @last_name)
  end
end

describe Person do
  subject { Person.new 'Josh', 'Cheek' }
  its(:name, '%f')    { should == 'Josh' }
  its(:name, '%l')    { should == 'Cheek' }
  its(:name, '%f %l') { should == 'Josh Cheek' }
end

This commit adds this feature.