Improve tokenization

[karlhorky]

As mentioned in #28 (comment), the tokenization and classes leave some things to be desired.

Would it fit the goals of this project to improve the support of the tokenization? I'm looking at this in terms of implementing more fully-featured themes, such as what I started in #28.

An example is this:

prism-react-renderer with Improved VS Code theme (#28)

VS Code

Would be nice to get a bit closer to VS Code's version (<, = and " punctuation highlighted differently).

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Looking more in detail, I see that there are some limitations inherent in Prism, but maybe there could still be some improvements somehow, such as integration of Prism JS Extras: PrismJS/prism#1743

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If improving tokenization is not easy or possible without changing Prism, one thing that could be easier to do would be to allow for changing the plain colors by language, so that the CSS plain could be the orange color instead of the light blue (which is more common throughout, which is why I opted to use it).

prism-react-renderer with Improved VS Code theme (#28)

VS Code

[karlhorky]

Also opened a request to improve the tokenization of Prism.js: PrismJS/prism#1871