Add a feature to insert headers and footers

[ElisabettaCarrara]

Add feature to allow using the plugin to modify headers and footers. Add a setting to toggle between simple mode (only headers and footers editing) and advanced (html/css/js/php snippet creation and usage). Add the chance to link the plugin to a snippet library (for example one could link it to its own github where snippets are located and can be copied and used).

This could be a way to implent the use of ClassicPress snippet directory in dashboard because the plugin could allow the sourcing of snippets from different origins (like the ClassicPress official repo for snippets or a repo/github account owned by a developper that creates such shortcodes).

[azurecurve]

Amending headers or footers would require php code to be written so would need to remain advanced mode. I could add an option to allow the load location of the php file to be set; default, header or footer.

I don't think hooking into repos for pulling snippets is going to be something I am able (or have time) to do. It had crossed my mind a while ago to allow export/import, but again not something I've had time to look into.