Add option solarized-scale-org-headlines

This adds an option to make it possible to disable the scaling of org-mode headlines but nothing else.

Starting to test out this theme, I found that I really liked the scaling of outline-levels in files like my init-file, but that it was too dominant in org-files, which I often use for outlining with wery little body text. That is, my org-files are almost all headlines, and this makes most text in org-buffers very big for me.

I thought this could be the case for others as well.