Issue title: Support Nim syntax highlight
 Issue body: [Nim](https://nim-lang.org/) is a programming language with about 5k stars. I want to be supported it.
 Comments: 
 Comment 0: To do that, write a CodeMirror mode and distribute it separately, on npm. We are not adding any new modes to the main distribution.
 Comment 1: Doesn't seem there's already one in existence at this point :/
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