New replacement pattern to pull metadata values in from the Data plugin.

Patterns are of the form: `@DATA:column@` where `column` is a Data-plugin key name as defined in the `----dataentry----` section of the page being exported to PDF.

This checks for the presence of the Data plugin, and does no replacements (but still works fine) if it's not found.

(I think this is a better idea than my earlier attempt (#22) at getting a document-specific 'revision-number' into the PDF footer. `:-)` Of course, I'm very open to be shown a better way!)