Use markdown-it instead of marked

[blu3r4y]

I know this is an opinionated PR, but, I propose to replace the renderer from marked to markdown-it.

Why?

• markdown-it is the second most-popular JS Markdown renderer (see here)
• markdown-it provides more features and easy extendibility via community-written plugins (see here)
• One useful plugin in particular would be markdown-it-attrs which allows one to add CSS classes, HTML attributes and identifiers to any Markdown element with this syntax: {.class #identifier attr=value attr2="spaced value"}
• Could resolve Add incremental reveal to slides #111 by adding {.fragment} class attributes along with scripts and styles (similar to cleaver-select-theme but without the need for explicit HTML)
• Could resolve Feature Request: support for math rendering #170 by adding markdown-it-mathjax
• Could resolve feature: add github-flavoured emojis #102 by adding markdown-it-emoji

Implications?

• The change is quite straight-forward, see attached code diff
• Shouldn't break existing presentations because markdown-it should be a superset to marked
• I also bumped the version of highlight.js in this PR just because I thought it would make sense as well