Make cover file optional

[dannystaple]

Instead of breaking here - make the cover file optional.

This fixes the error seen below when a cover file isn't present:

/srv/jekyll/_plugins/thumbnail_generator.rb:78:in `read': unable to open image 'assets/posts/foam-core-drawer-dividers/cover.jpg': No such file or directory @ error/blob.c/OpenBlob/3496 (Magick::ImageMagickError)

[oturpe]

Makes sense, not every site needs to have a cover for every doc. However, in the case where every doc is supposed to have a cover, it is useful if not having it alerts the user somehow during build. Crash and a stack trace is not the best way, but a warning would be useful. So, to cover both cases, perhaps something like this: A new entry for configuration option thumbnail: boolean required. If set to true, prints warning every time if cover is not found. If set to false, only prints a warning if the cover was explicitly set in the front matter, but still is was not found.

What do you think?

[dannystaple]

I think that makes a lot of sense - make it optional, like a compiler warning flag.