feature(plugin): page collection preprocessing

Adding a pre processing plugin to allow adding properties to
pages in the pages collection.

feature(plugin): page collection preprocessing

Adding a couple of preprocessing functions for testing the
page collection preprocessing plugin.

feature(plugins): page collection preprocessing

Adding the default isActive and relativeLink property functions
to the plugin itself.
Merge pull request #413 from doowb/plugin-page-collection-properties

feature(plugin): page collection preprocessing