page2svg: add a --option flag

This option file will point to a .js file. The js file must exports a config key containing MathJax options (what you would put in MathJax.Hub.Config)

I use this when I want to use MathJax on a HTML file setting MathJax options with MathJax.Hub.Config ; the configuration file looks like, for example : 

     exports.config = ({
        TeX: {
          equationNumbers: {
            autoNumber: "AMS", formatNumber: function (n) { return "8." + n }
          },
          Macros: {
            fournabla: ["\\nabla\\!_\\mu", 0]
          }
        }
      });