Preserve ZLE_KILL and ZLE_YANK flags

[macdems]

This solves #363.

[vincentbernat]

I am running into an odd bug when applying this commit. Steps to reproduce:

1. Start a fresh shell
2. Type echo hello world bye
3. Type Alt-Backspace (this deletes bye and put it in the ring)
4. Type Backspace (this deletes the space)
5. Type Alt-Backspace (this deletes world and put it in the ring)
6. Type Alt-b (go at the beginning of hello)
7. Type Ctrl-Y (paste world)
8. Type Alt-Y (should replace world by bye)

What I get is:

echo hbyeello

Instead of:

echo byehello

ZSH 5.8. Do you have the same bug?

[aculich]

@vincentbernat I added your suggestion as a test and opened a new PR #795 that builds on this one... hopefully they'll both be merged soon @macdems ?