add on page change hook

[dtnaughton]

• Added onPageChange hook which fires on router path change.
• Function is passed into plugin as an argument. If no function is passed, generic blank function is set.
• Function currently gets args from the plugin itself (search query, current path and next path). Sufficient for now but could be made more generic by not relying on these args.

[jenessaman]

Implementation is based on: z3by#37 (@dtnaughton - its worth putting a link in a comment, so we can refer back to this original issue and basis for implementation if needed)

[dtnaughton]

closed PR because original branch was accidentally deleted. Opened a new PR which captures all changes in this PR and additional changes related to these comments #2