[FR] Fontifying src-blocks with no language specified

[carsten@kragelund.me]

Fontifying src-blocks with a missing language would allow being able to 
theme src-blocks without having the unthemed text of languages fallback 
to whatever color the src-blocks are set to.
I envision this could be done with an empty string in 
org-src-block-faces.

See the below example where printf has the org-block face, in this case 
it would allow using a more normal color by default for org-blocks where 
the blocks without a language could use a more pronounced color.

#+BEGIN_SRC C
int main() {
     printf("Hello World\\n");
}
#+END_SRC

#+BEGIN_SRC
this is just a block of text
#+END_SRC