EscapeHTML function

EscapeHTML function

Another function to encode html/xml, this time (imho) the "right" way by
only quoting reserved characters.  The provided Entity() function would
often be unusable because of its overcomplete whitespace formatting.
This is a feature which I (have to) set up manually in many cases, which
seems very unfriendly for a module optimised for outputting HTML.

According to personal preferences, it only substitutes a minimal set of
entities:
- & and < (both required to prevent html interpretation)
- > (for xml or otherwise to ease document parsing)
- " (to make it usable in attribute values).

Single quotes (' or ') are left unquoted, assuming attributes are
always in double quotes (no reason to do otherwise).
Unlike Entity, it only handles a single argument, to allow for possible
options in the future (hopefully supporting a custom range of unsafe chars).
It also dies on failure (like trying to change read-only input), because
that is a user mistake which should not go unnoticed.

The name was devised to be more consistent with other environments (also
anticipating new URI encoding and decoding):

* php                   htmlspecialchars  html_entity_decode urlrawencode       urldecode
* javascript                                                 encodeURIComponent decodeURIComponent
* ruby CGI              escapeHTML        unescapeHTML       escape             unescape
- CGI::Simple::Util     escapeHTML        unescapeHTML       escape             unescape
- CGI::Util             (simple_escape)                      escape             unescape
- HTML::Mason::Escapes  basic_html_escape                    url_escape
- HTML::Tiny            entity_encode                        url_encode         url_decode
* URI::Escape                                                uri_escape_utf8    uri_unescape
* XML::Quote            xml_quote         xml_dequote
- PLP (legacy)          Entity                               EncodeURI          DecodeURI
- PLP (redesign)        EscapeHTML        UnescapeHTML       EscapeURI          UnescapeURI

HTML:
- Escape etc used nearly everywhere (so the obvious choice).
- Decode is only used by php, but uglily and inconsistently.
- Quote seems most appropriate linguistically, but only used in one minority
  module.

URIs:
- Encode etc common in php and javascript.
- Escape etc used by ruby and several perl modules (including URI::Escape),
  and is still familiar to javascript users
- URI used in all significant environemnts; URL only in minor modules.