(Feature) : support sitemesh as template engine

[supsup]

would be nice to have template engine choices:

https://github.com/sitemesh/sitemesh2
https://github.com/sitemesh/sitemesh3

I saw in issue:
#228

from @manikmagar
that :

I haven't used Rythm but in general, it will always be good to support various templates. As of now, all parser engines are part of core functionality. Adding a new parser means a change in the core. I believe as a part of plugin discussions, it was discussed that we should probably extract out the parser engines so that they can be implemented independent of core jbake and injected as a plugin/extension.

Was the idea ever done?