XML identifier compatible base64 variants

[jpetso]

Wikipedia mentions two base64 variants for use in XML identifiers:

• a variant using .- for use in XML name tokens (nmtoken) - likely cppcodec name: base64_xml_nmtoken
• a variant using _: for use in more restricted XML identifiers (name) - likely cppcodec name: base64_xml_name.

These don't seem to be specified in any high-profile standard, or I missed it. The only reference to base64 in XML I found with a quick Google search was the XML Schema type base64Binary, which uses newline-optional MIME base64 and thus isn't XML-specific.

So these are presumably not in use all that much, and probably super low on the list of codecs to add. If anyone would like to see either of these variants in cppcodec, please comment on this bug.