Ignore case of the language name

[regseb]

Hi,

• Prism is case insensitive: https://jsbin.com/setahaweda/edit?html,output
• But prism-react-renderer is case sensitive: https://codesandbox.io/s/prism-react-renderer-example-forked-ogm40y

It would be nice if prism-react-renderer ignored case (to have the same sensitivity as Prism).

[regseb]

The problem has been fixed (probably thanks to the #191 pull request): https://codesandbox.io/s/prism-react-renderer-example-forked-ppy1z7