add Context

[lain0]

so it can be used like:

BDDSM.describe '.context' do
  let(:user) { create(:user) }

  context 'when user is valid' do
    it { expected(user).to be valid }
  end

   context 'when user is invalid' do
     let(:user) { User.new }

     it { expected(user).to be invalid }
   end
end