Option to render out HTML styling rather than ANSI escape codes

[obiwac]

Useful for displaying logs in webpages or for use by applications which use aquabsd.alps.font to render logs in text (which in turn uses Pango which uses a HTML-like markdown language for colouring).

Could be implemented as either a function programs using Umber can call, either as an environment variable (like UMBER_HTML_OUTPUT=1).