Add a "use_rspec_syntax" configuration option.

Allows users to do

`configuration.use_rspec_syntax(:expect)`

instead of:

```
  config.mock_with :rspec do |configuration|
    configuration.syntax = :expect
  end

  config.expect_with :rspec do |configuration|
    configuration.syntax = :expect
  end
```

I briefly told @alindeman about this when we were pairing and he said he thought it was a good idea :)