Enable definition lists

[spectria-limina]

Pullmark now supports definition lists with the ENABLE_DEFINITION_LIST option. Please enable this for Zola.

[Keats]

Could be added via a setting in config.toml i think

[matthiasschaub]

I was just about to ask to enable definition lists as well!
The PR which adds support for definition lists is https://github.com/pulldown-cmark/pulldown-cmark/pull/915/files

I use definition lists extensively, and it would be great if Zola supports it.

By the way, I really love Zola and appreciate the simplicity. Thanks for creating and maintaining Zola!