New: textproc/ruby-herb

I'd like to add a port for ruby-herb. This is a Ruby library that
implements a combined HTML+ERB parser, which allows it to have
features that normal ERB parsers do not, such as checking the HTML
for validity while parsing the ERB template, and potentially
optimizing the generated template code using inlining.

It includes a native extension, so it has a benefit to being packaged.

Tested on amd64.

OKs to import?

Best,
Jeremy