Load plugin JS after native JS to enhance plugin capabilities

Idea from the discussion at pull request <https://github.com/gothfox/Tiny-Tiny-RSS/pull/110>.

By loading plugin JS after native JS, plugins have the ability to change variables (and perhaps override functions, though I haven't tried that) to define e.g. new hotkey actions.

I've tried this without issue so far. I can't personally see any potential problems or any functionality that could be dependent on plugins being loaded first.