Function: to_html

--Usage--
> to_html(some_tagged_text)

Convert a [[type:tagged string]] to HTML code for use in web pages.
Characters are correctly escaped for HTML code.

The following tags are converted to html:
! Tag			Description
| <tt>&lt;b></tt>	Bold
| <tt>&lt;i></tt>	Italic
| <tt>&lt;sym></tt>	Symbols, if the @symbol_font@ parameter is set.

Symbol fonts 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]] (optional)	Name of a symbol font to use for images.
| @symbol_font_size@	[[type:double]] (optional)	Size in points to use for the symbol font, default 12

--Examples--
> to_html("<b>bold text</b>") == "<b>bold text</b>"
> to_html("<sym>WU</sym>")    == "<img src='data-dir/w.png' alt='W'><img src='data-dir/u.png' alt='U'>"

--See also--
| [[fun:symbols_to_html]]	Convert text to html using a [[type:symbol font]].
| [[fun:to_text]]		Remove all tags from tagged text, and convert it to a [[type:string]].