format_* / rendering plugins

It is rather simple to add WikiMarkup using the $ewiki_config["wm_..."]
settings, but for some tasks you need stronger weapons like rendering
and markup plugins.

The ewiki_format() function (often blatantly referred to as "rendering
kernel") recently got rather complicated to add support for the 'block'
plugins. Therefore we'll first need to discuss the variables structures
and names used inside of it:

prev << "writing your own plugin"
next >> "ewiki_format() internals"

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You cannot modify the INTERNALS file, but anyhow any ideas or suggestion should as usually get filed on BugReports, UserSuggestions or even better the INTERNALS.Discussion.