textproc/R-cran-tth: New port: Convert TeX/LaTeX to HTML/MathML

C source code and R wrappers for the tth/ttm TeX-to-HTML/MathML translators.

The R package tth ships the C sources for convenient compilation and
installation on all platforms.
It also provides wrappers in R to process R character vectors with the
command line tools directly from the R prompt.

The textproc/tth port ships `tth' binary but the two sources differ.
The `ttm' binary isn't available in the ports tree.

Approved by: diizzy (mentor)
Differential Revision: https://reviews.freebsd.org/D44696