tex-converter -- transform foo.ad into foo.tex

This is a start on a backend to convert Asciidoc files to LaTeX files.  The README and the comments in the code say more.  Included are `sample1.ad`, `sample2.ad` and `elliptic.ad`which test the converter.  Run with
```
$ asciidoctor -r ./tex-converter.rb -b latex sample1.ad -o sample1.tex
```
etc.