Support some kind of export format like XWR to be able import data into another CMS

[gerdneuman]

Spawned from #236 (comment)

What is really needed is something more like WordPress's XWR format that exports the content but not the internal structure of a particular blog, so it is easy to move content from one instance of the WordPress software to another, independent of the underlying database.

Currently user are "trapped" to using crabgrass.

@meitar cc