Add 'OnMODXInit' plugin event

(cherry picked from commit 40429cf199355294ae293e73455b371f4fdc4edb)
Add contextKey, options as OnMODXInit params
Add changelog entry
Add OnMODXInit event in modX::initialize()

Merge branch 'OnMODXInit' into develop

* OnMODXInit:
  Add changelog entry
  Add contextKey, options as OnMODXInit params
  Add 'OnMODXInit' plugin event