added ability to customize localIdentName without ejecting

Created by: anatolzak

I added the ability to customize the css/sass localIdentName without the need to eject.

I always eject my projects simply because I only want to customize localIdentName, which seems unnecessary to me. So I fixed it with this pull request :)

Now, you can simply add the variable LOCAL_IDENT_NAME to a .env file to customize the localIdentName.

Here is an example of my .env file:

LOCAL_IDENT_NAME=[hash:base64:5]

So now my css classes look like this:

._3iokT {
    font-weight: 500;
    font-size: 15px;
}

instead of like this:

.Card_Title__2DIe2 {
    font-weight: 500;
    font-size: 15px;
}

This also allows to reduce the css and js file bundle since we can make the css classes shorter.

All of this is possible now without needing to eject!

I will greatly appreciate if you consider this pull request! Let me know if you have any questions!