Function: symbols_to_html

--Usage--
> symbols_to_html(some_tagged_text)

Convert a [[type:tagged string]] to HTML code for use in web pages, entirely using a symbol font.
This is intended for fields with @always symbol:true@.

This function can only be used in an [[type:export template]], when <tt>create directory</tt> is true, as the images
of the font are written to the output directory.

--Parameters--
! Parameter		Type				Description
| @input@		[[type:tagged string]]		String to convert to html
| @symbol_font@		[[type:string]]			Name of a symbol font to use.
| @symbol_font_size@	[[type:double]] (optional)	Size in points to use for the symbol font, default 12

--Examples--
> symbols_to_html("WU")    == "<img src='data-dir/w.png' alt='W'><img src='data-dir/u.png' alt='U'>"

--See also--
| [[fun:to_html]]		Convert [[type:tagged text]] to html.