Allow loading additional syntaxes for code highlighting

[phil-opp]

The syntect crate ships a default set of syntaxes that allows to highlight most common languages, but it does not include syntax files for more exotic languages such as nasm. It is possible to load additional syntaxes for such languages through the SyntaxSet::load_syntaxes method. It would be great if gutenberg could support this somehow, for example by adding support for all needed languages in gutenberg itself or by adding a command line argument.

[Keats]

I guess that means keeping track of the official repo and some packages like https://packagecontrol.io/packages/NASM%20x86%20Assembly in your case and LESS/SCSS/Jade/etc that Sublime Text users typically need

I'll have a look today or tomorrow