We can't always tell if it's LaTeX, ConTeXt, or plain TeX.
Better just to use "tex" always.
Also changed:
ConTeXt writer: now outputs raw "tex" blocks as well as "context".
(Closes#969).
RST writer: uses ".. raw:: latex" for "tex" content.
(RST doesn't support raw context anyway.)
Note that if "context" or "latex" specifically is desired,
you can still force that in a markdown document by using
the raw attribute (see MANUAL.txt):
```{=latex}
\foo
```
Note that this change may affect some filters, if they assume that raw
tex parsed by the Markdown reader will be RawBlock (Format "latex").
In most cases it should be trivial to modify the filters to accept
"tex" as well.
