Support conversion of pending examples

[yujinakayama]

• Feature request: shortcut for pending-block within it rspec/rspec-core#1208
• Mark pending blocks as failed if they succeed. rspec/rspec-core#1267
• Provide upgrade warning for pending behaviour change. rspec/rspec-core#1268

Convert:

describe 'RSpec 2' do
  # Skipped
  pending 'does something' do
    do_something_possibly_fail
  end

  # Skipped
  it 'does something' do
    pending
    do_something_possibly_fail
  end

  # Skipped
  it 'does something', :pending do
    do_something_possibly_fail
  end

  it 'does something' do
    pending do
      # Run and expected to fail
      do_something_surely_fail
    end
  end
end

To:

describe 'RSpec 3' do
  skip 'does something' do
    do_something_possibly_fail
  end

  it 'does something' do
    skip
    do_something_possibly_fail
  end

  it 'does something', :skip do
    do_something_possibly_fail
  end

  it 'does something' do
    # #pending with block is no longer supported
    pending
    do_something_surely_fail
  end
end