Add Git-LFS support to the GitHub backend

Thank you for the Netlify CMS. I am experimenting with it at the moment and I am frequently delighted as I explore the way it works!

**- Do you want to request a *feature* or report a *bug*?**
Feature request

**- What is the current behavior?**
When using the Netlify CMS media browser for image selection it displays empty thumbnails

**- If the current behavior is a bug, please provide the steps to reproduce.**
When using the Netlify CMS for image selection the media browser normally displays thumbnails of uploaded images to select from.

When LFS (large file support) is enabled the media browser displays empty thumbnails.

**- What is the expected behavior?**
I would like Netlify CMS media_folder browser to display thumbnails when LFS is enabled for a repo

**-Additional info**
If viewing the url of one of the broken thumbnails it is the path to the 'text pointer' for the image file:
```
version https://git-lfs.github.com/spec/v1
oid sha256:e57917d40d1273633a8ce5d52e14c2534b85cfd289dc2e89a048a6b888eb214c
size 29860
```
Images that are placed on the page using this method do work correctly and are deployed successfully, it is only the media browser thumbnail that is broken.

Below is a picture showing what happens when the media_folder browser is used on an LFS image. As you can see in the background, an image that has been chosen renders correctly, but the thumbnails in the browser do not display.

The repo this example is taken from is at