markup plugins

The ewiki rendering core is rather fast and consolidated, that was the goal.
However if you ever happen to need more functionality, this can be added
easily by the use of plugins.

Several are already available to emulate the WikiMarkup of other commonly
used WikiWare+.
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You cannot modify the README.plugins file, but anyhow any ideas or suggestion should as usually get filed on BugReports, UserSuggestions or even better the README.plugins.Discussion.