tex-converter.rb

This is a start on a latex backend for asciidoctor.  Run via

```
$ asciidoctor -r ./tex-converter.rb -b latex sample1.ad -o sample1.tex
```
This gives the core of the needed tex file.  To get a complete working example, you need
the files `sample1.ad`, `preamble`, and the shell script `run.sh`, all of which I will attach.
Then

```
$ sh run.sh sample1
```

will produce `sample1.tex`.  It can be processed to give a pdf file via `pdflatex sample1.tex`.