Preparing to increase format of output other than html.

## Motivation

GH-33 and GH-48

## Work

Split `RSpec::Core::Formatters::TurnipFormatter` into following classes:

- `RSpec::Core::Formatters::TurnipBaseFormatter`
- `RSpec::Core::Formatters::TurnipHtmlFormatter`

```ruby
module RSpec
  module Core
    module Formatters
      class TurnipHtmlFormatter < TurnipBaseFormatter
      end
    end
  end
end

RSpecTurnipHtmlFormatter = RSpec::Core::Formatters::TurnipHtmlFormatter
RSpecTurnipFormatter = RSpec::Core::Formatters::TurnipHtmlFormatter # For compatibility
```

## Future

```ruby
module RSpec
  module Core
    module Formatters
      # GH-48
      class TurnipJsonFormatter < TurnipBaseFormatter
      end

      # GH-33
      class TurnipTextFormatter < TurnipBaseFormatter
      end
    end
  end
end
```