MATHJax font options for Orca

[ashwin-nayak]

I had to use plotly's graphs for publications and some of them contained TeX-based text. Plotly's graph font options and TeX font options were always mismatching and thicker in strokes, even while I knew MathJAX had the capabilities to publish in Modern Math or few others.

MathJAX's SVG output processor provides certain font options. By not selecting explicitly earlier, Orca would output images in STIX-Web font with the default blacker:1 attribute which renders a slight difference for TeX-rendered fonts compared to the overall Graph fonts.

Included are a couple lines of code for constants.js and inject.js for including these choices 'explicitly' to enable changing the fonts by changing a parameter in constants.js manually. Future feature requests may consider exposing this to the plotly export feature, if at all.