Investigate API to hook into user scripts loading

[tmedwards]

Investigate adding an API to allow hooking into the user scripts load sequence to allow custom special tags with associated behaviors.

E.g.,

_CustomTagAPI_.add('userscript', function (passage) {
    /* Process `passage` as a script. */
});
_CustomTagAPI_.add('userstyle', function (passage) {
    /* Process `passage` as a stylesheet. */
});
_CustomTagAPI_.add('something', function (passage) {
    /* Process `passage` in some way. */
});

This can already be done with JavaScript alone, so this isn't a necessity, but it might be nice to have.

[hituro]

It would likely be cleaner than the various JS solutions if only because there are nuances to the processing of special elements (e.g. the IMG markup in stylesheets) that could otherwise be left out.