Async right prompt

[ayyess]

This could be a solution to the lag experienced in #244.
An subprocess writes the prompt to a temporary file (unique to each zsh instance) as each element is loaded. A signal is send to the parent zsh process after each write so that the RPROMPT can be set to the new value. The temporary file is deleted when the zsh exits.

The three zle reset-prompt were removed from the other handlers as otherwise RPROMPT was reset to empty after hitting enter. However powerlevel9k_prepare_prompts handles zle reset-prompt so this shouldn't be a problem

[Tritlo]

Please make a pull request against next, that's where new stuff should come in.

That being said, when I try to run this I get:

remove /var/folders/wr/fk_4qbzn72x34r7xgs173d_40000gn/T/tmp.g0ohipLH? y
async:1: no such file or directory:
build_right_prompt:13: no such file or directory:
build_right_prompt:13: no such file or directory:

I am on os x, loading with zgen and zpresto.

[Tritlo]

Alright, I've forked your fork and fixed it for me on os x, see ayyess#1. Works great, though it is a bit annoying that it doesn't just wait for all the parts, but rather renders them as they come. Would be really nice to be able to just wait for segments to finish, and then render.

If you move the kill -S USR1 $$ outside of the loop, it waits until the entire segment is done rendering.

Edit: I've done that on my fork, and I think it looks a lot smoother.

[ayyess]

I don't seem to be able to change the target branch of this pull so should I make a new pull request?
New request at #263