Use locale as root key in translation source files

[zedissime]

I'm using a translation tool which generates me my translation files. It generates my files but he uses the locale as the root key of my translations : For en.yml

  en: 
    title: Hello
    ...

and for fr.yml

  fr: 
    title: Bonjour
    ...

So I propose to add an option to pass in order to handle this.

[danhper]

Looks good, thanks!

[danhper]

I just published a new version with your PR.