Refactor code to allow TikZ to html conversions on demand

[rei2hu]

This is a refactoring that will allow users to perform conversions on demand, keeps the existing functionality, and fixes some bugs.

I had a use case where I wanted to do the main processing in a Worker, so I made these changes (and some more that aren't included) to support that. I've left most of the existing code intact. Also helps with #8, #20, and #24.

TikZJax.js contains all of the conversion logic and exposes a load(wasmPath, coredumpPath) method that can be called to instantiate the wasm module on demand.

index.js contains all of the logic that relies on being in the dom.

I've updated the README with a basic example of how to load things manually.

I also had to pull in a different version of dvi2html to get things to build, but that should not be an issue because the interface looks to be the same.

The repository seems incomplete so let me know if I've messed up some hidden workflow.

[rei2hu]

I'm not sure if this is considered a bug or not, but TikZJax was not being executed with the onload function, but was being executed when the line was hit (because the result of the call is what's being assigned to window.onload)

[rei2hu]

load returns a function that sets up wasm with the loaded payload/core dumps, so theoretically you could load different wasm/coredumps.

const tex1 = await load(wasm1, coredump1);
const tex2 = await load(wasm2, coredump2);

tex1(tikz1);
tex2(tikz2);

The main point of this is to prevent loading multiple times though. I don't see any benefit to loading multiple times.