* htmlfontify.el: Add support for code block fontification

* htmlfontify.el: Add support for code block fontification
for ODT export.
(hfy-optimisations): Define new option
`body-text-only'
(hfy-fontify-buffer): Honor above setting.
(hfy-begin-span, hfy-end-span): New routines factored out form
`hfy-fontify-buffer'.
(hfy-begin-span-handler, hfy-end-span-handler): New variables
that permit insertion of custom tags.
(hfy-fontify-buffer): Use above handlers.
(hfy-face-to-css-default): Same as the earlier `hfy-face-to-css'.
(hfy-face-to-css): Re-defined to be a variable.
(hfy-compile-stylesheet): Modified.  Allow stylesheet to be built
over multiple runs. This is made possible by having the caller let
bind a special variable `hfy-user-sheet-assoc'.
(htmlfontify-string): New defun.
(hfy-compile-face-map): Make sure that the last char in the
buffer is correctly fontified.
(hfy-face-resolve-face): Whitespace only change.

Fixes: debbugs:9914