- Added extension ent2uni, a postprocessor which maps HTML entities to unicode

The post processor maps HTML entities to their unicode equivalents.

This was needed to sanitize output from the Asciidoctor LaTeX  backend before running it through `xelatex`, but it is perhaps useful for other things as well. 

Current workflow for `.adoc` to `.tex` using the normal math convention $ a^2 + b^2 = c^2 $ is 

```
$ a2l foo.adoc
```

where

```
alias a2l="$adb/asciidoctor -r $pre -r $post -r $lco -b latex"
```

and where `$pre`, `$post`, and `$lco` point to `tex-preprocessor.rb`, `ent2uni.rb`, and `asciidoctor-latex/converter.rb`.