Elm syntax highlighting with PrismJS

[kachkaev]

Prism now supports syntax highlighting for elm, thanks to PrismJS/prism#1174. This makes it possible to highlight elm code in the markdown output, which is what I implemented in this PR.

Because mume partially relies on dependencies/*.js instead of node_modules, here are the details of how I updated dependencies/prism.js:

1. Went to http://prismjs.com/download.html
2. Selected all languages
3. Downloaded the js file and saved it as dependencies/prism.js
4. In a rather large diff, searched for a line starting with Prism.languages.elm= and staged it

This means that PrismJS has not been updated as a whole. Although this ensures that mume does not break with this PR, it'd be great to generally keep in sync with prism as it gets updated. @shd101wyy could you please briefly describe how to generate dependencies/prism.js from source inside dependencies/README.md?

To be honest, I'd prefer to just sticking with node_modules, but I imagine there's been a really good reason to switch to a custom dependency management. Just curious: in which scenarios large node_modules ended up being a pain for mume? Can't remember that in other Node.js (i.e. not-in-browser) projects.

[shd101wyy]

Hi @kachkaev , I didn't choose node_modules for some packages because I had trouble packaging vscode extension. If I recall correctly, vscode extension has a size limit of 20mb. There are also some reasons but I don't remember exactly.

I will have a look at your pull request when I have time.

Thank you very much for your contribution.

[shd101wyy]

Looks good to me. You can merge it at anytime. Thanks.

[kachkaev]

Thanks @shd101wyy! Would you like me to prepare a new release? I can't publish on npm, but could change the version, update changelog etc. Happy if you do that too, just asking because I know you're a bit busy now

[shd101wyy]

Thanks @kachkaev ! I will finish reviewing your pull requset #34 and #31 by the end of this week. Then we can publish a new version.