[conspire] Automatically doing em dashes in Vim?

[Don Marti]

Here's a Vim question. Basically I want the semicolon key to act like an 
em dash key where it makes sense—when I'm editing a file that's mostly 
text and putting the punctuation mark between two word characters. 
(Semicolons are generally for code, not for prose.)

If I'm editing an HTML or Markdown/CommonMark file, and I type a word 
character followed by a semicolon followed by another word character, 
what I want instead of the semicolon is an em dash (either the HTML 
— or the Unicode —  I'm not picky.) Semicolons at the end of a 
line or followed by a space should be left unchanged.

Is this doable? Pretty sure it should be but can't find anything 
directly relevant.