new plugin: latex_reader

This plugin installs the TeXReader reader
which can read most (La)TeX documents.
latex_reader: check if HTML produced instead of exit code

due to the complicated pipeline the exit code is not reliable
latex_reader: set TEXINPUTS to enable searching for content

this mainly helps \input and \include* macros
also add future plans
latex_reader: better info about using MathJax
latex_reader: better support for images (width)
latex_reader: support gif width