Make combinerMJlist more configurable

See also this pull request on MathJax:  https://github.com/mathjax/MathJax/pull/433

This change lets me use combineMJlist outside of a MathJax project directory, to create my own custom combined configuration file.

For example, I use it with the command line

    $MJXDEV/combineMJlist $MJX mathjax-config-pubreader.lis .

The output lands in the current directory (".")

My ".lis" file is modified such that the first line gives the value to put in the "load complete" call at the end of the generated config file, and looks like this:

    LOAD-COMPLETE-PATH: /corehtml/pmc/js/mathjax-config-pubreader.js
    jax/input/MathML/config.js
    jax/output/SVG/config.js
    ...
    jax/output/SVG/autoload/mtable.js

        SVG: {
            scale: 90,
            linebreaks: {
                automatic: true,
                width: "container"
            }
        },
        showProcessingMessages: false,
        MathZoom: {
          styles: {
            "#MathJax_Zoom": {
              "background-color": "#eee",
              "position": "static",
              "overflow": "auto !important",
            },
            "#MathJax_ZoomFrame": {
              "max-width": "95%",
              "display": "block",
              "width": "99in",
              "left": "8px",
              "z-index": "1"
            }
          }
        }