Run OhMyREPL keybindings in a fixed world age

[c42f]

Hopefully this should avoid most invalidations caused by loading other packages as discussed in https://github.com/JuliaLang/JuliaSyntax.jl/issues/252.

Also refactor/replace the NEW_KEYBINDINGS global as it's hard to follow where this is added to, and surface a bunch of other uses of global variables like Base.active_repl up to the top level init.

I'm not sure whether there's some additional wrapping which is needed around the pirating in refresh_lines.jl, etc?

Seems to fix #316 ?

@jakobnissen You've used OhMyREPL a lot more than me, perhaps you could give this a test drive? I'm not entirely sure of all the ways I should be trying to test this.