code for showhtml configuration option needs to be improved or deprecated

[jkbzh]

This option allows to show the articles in a proportionally-spaced
font rather than a fixed-width (monospace) font or do a more complex conversion to HTML.
The code that does this has not been reviewed since long time ago and needs someone to maintain and improve it.
Lack of either interest or maintenance for this option will put it at risk of deprecation in the next version of hypermail.