There is no reason to limit it to "xhtml" only. Even more, in all
browsers I tried, the exported page was failing to load with the '.xtml'
extension, while it loads fine when renamed as '.html'.
Only problem is that now we have 2 plug-ins able to save as html (other
is file-html-table) and the first in the procedure list (with no
explicit logics right now) will just shadow the next one. We will have
to add some generic infrastructure to allow people to choose favorite
load/export plug-ins, probably in Preferences, next to the raw plug-in
selection.
