--config toml/yaml option for jekyll new command

Create new project with __.toml__ config file
```
jekyll new testblog --config toml 
```
Create new project with __.yml__ config file
```
jekyll new testblog --config yaml
jekyll new testblog --config yml
jekyll new testblog
```
Of'course, you can use shorcut ```-c``` instead ```--config```
Can you please provide the default?