Ordering & "after/"

[SevereOverfl0w]

Zen doesn't include the after/ directory from a plugin on the rtp.

I looked into adding this but zen appends directly onto the rtp.
I've done some follow-up research into this, and discovered junegunn/vim-plug#615.
The linked issues inside are also relevant.

The summary is that ordering is really important with the rtp.
The order is:

• original rtp at beginning, as spellfile writes there by default
• pluginA, pluginB
• pluginA/after, pluginB/after
• end of rtp to begin with (home/dominic/.config/nvim/after for me) to allow users to override settings from plugins

These are just things I figured out from looking at vim-plug.
It might be more insightful to look at vim8's packaging feature, as that performs this ordering.
vim8 is more likely to be a canonical source of information.

These options I've been able to come up with:

• It is possible to match the ordering of vim-plug by using splitting on the rtp.
  Junegunn hasn't looked into the performance impacts of that though, it may not be viable.
• Convert &rtp at first into a list, and then on every zen#add call, update both the list and the 'rtp' setting. (maybe with a function to indicate manual update to &rtp?)
• Switch to a vim-plug zen#begin and zen#end, but if you wanted to do that you'd already be using vim-plug I'd bet!
• Integrate with vim8's package feature, and let that take care of ordering.
  This would make the plugin be closer to minpac's current feature set.