Add CHTML Output Processor

[tani]

Closes GH-42

I add two plugins rehypeMathJaxCHTML and rehypeMathJaxSVG instead of rehypeMathJax.
Users can choose the output processor of MathJax by this change.

API

const rehypeMathJaxCHTML = require('rehype-mathjax/chtml') // Import CommonHTML ouput processor
// const rehypeMathJaxSVG = require('rehype-mathjax/svg') // Import SVG output processor
// const {rehypeMathJaxCHTML, rehypeMathJaxSVG}  = require('rehype-mathjax') // Import both processors
unified().use(rehypeMathJaxCHMTL, options)

• options: MathJax ouput options for CommonHTML ouput processor and SVG ouput processor

[wooorm]

Nice! 🎊

• I’d prefer casing acronyms as normal words: CHTMLRenderer -> ChtmlRenderer, fontURL -> fontUrl, rehypeMathJaxSVG -> rehypeMathJaxSvg; what do you think?
• I think it makes more sense to default to one from index.js instead of exporting both. Exporting both is probably not what you want. We should pick the one that makes the most sense and export that from index.js.

[tani]

I updated symbol names, however, we should not change the symbol fontURL because this is not defined in our code. This symbol named by upstream, MathJax.

[tani]

According to the documet of MathJax,

Currently, MathJax can render math in three ways:

Using HTML and CSS to lay out the mathematics,
Using Scalable Vector Graphics (SVG) to lay out the mathematics, or
As a serialized MathML string.
The first two are implemented by the CommonHTML and SVG output processors. The third is a consequence of the fact that MathJax uses MathML as its internal format. While MathJax version 2 included a NativeMML output processor that produced MathML notation for those browsers that support it, this has been dropped from version 3. See the MathML Support section for more information on how to get MathML output.
Currently, MathJax can render math in three ways:

Using HTML and CSS to lay out the mathematics,
Using Scalable Vector Graphics (SVG) to lay out the mathematics, or
As a serialized MathML string.
The first two are implemented by the CommonHTML and SVG output processors. The third is a consequence of the fact that MathJax uses MathML as its internal format. While MathJax version 2 included a NativeMML output processor that produced MathML notation for those browsers that support it, this has been dropped from version 3. See the MathML Support section for more information on how to get MathML output.

If you are using one of the combined component files, then this will select one of these output processors for you. If the component file ends in -chtml, then it is the CommonHTML output processor, while if it ends in -svg then the SVG output processor will be used.
If you are using one of the combined component files, then this will select one of these output processors for you. If the component file ends in -chtml, then it is the CommonHTML output processor, while if it ends in -svg then the SVG output processor will be used.

There is no recommendation of output format, every output format treated equally in document.
As SVG output format has no external dependency, I prefer to set it default.
While we have to set fontURL when we use CHTML output format, this is better to reduce the bundled size. I'd like to avoid the external dependency even if the bundle size is reduced. How about you?

[wooorm]

Yeah, seems like SVG is a good default, as it works without options!