Insert em-dashes when there are three hyphens

[mathisonian]

Is your feature request related to a problem? Please describe.
It is common for CMSs to automatically replace three hyphens (e.g. ---) with an em-dash (—).

Describe the solution you'd like
Similar to how Idyll will automatically insert smart quotes, it should also convert hyphens to em-dashes, possibly with an option for users to opt-in/out of this behavior.

This logic should be added to the compiler in the same spot the smartquote logic currently exists (https://github.com/idyll-lang/idyll/blob/master/packages/idyll-compiler/src/processors/post.js#L26-L32).

This can be added via a compiler plugin. It should be added to the list of default plugins; the smartquote plugin would be a good starting point to copy, as that also modifies an article's text.

[jimidle]

I was going to have a go at this Matthew, but it seems that post.js is no longer in existence. I will read through the source of the compiler and see if I can make sense of the change logs etc. But if you have time to update the information about this issue, then it may make things a little quicker.

It looks like you moved to a set of plugins in the compiler, and so seemingly the correct way to this, would also be via a plugin?

[mathisonian]

Thanks @jimidle! Yes - implementing this via a plugin makes sense, I just edited the issue to reflect the recent changes