Front-End Image Uploads

[JayWood]

@jtsternberg A few days ago I ran into the need to get this working in this commit - 8cd6c29

Obviously manage_options was too high, but upload_files is all that's really needed.

However, further exploration into this, if a post is published and the user is creating a 'front-end' edit form of some fashion, we also need to include 'edit_published_posts' as a cap. However this was really an edge case due to me needing to use 'contributors' and not subscribers.

So, the question is actually should we document this stipulation somewhere like here:
https://github.com/WebDevStudios/CMB2-Snippet-Library/blob/master/front-end/cmb2-front-end-wordpress-media-uploader.php#L29

Or just ignore it since it's an edge case...