Implement a "Update content" mojo

The use case is the following : when building a wiki that depends on another wiki, being able to affect the content of wikis page from the depending wiki.
For example, appending an include at the end of the content of a wiki page, to enrich this page, and still be able to benefit from its evolutions (which would not be the case just overriding the wiki document).

Proposed properties :

• content (text)
• append (boolean, if true, append the content to the existing content, if false, overrides the whole content)