add positional completion to issue edit

[claytonrcarter]

#245 landed just after #252, so it missed the completion party. I think that this adds the required code to enable positional completion of [remote] and <id>. @rsteube can you take a peek to make sure I'm doing it right? I copi...based this off of the completions in issue note.

Thanks!

[codecov]

Codecov Report

Merging #254 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #254      +/-   ##
==========================================
+ Coverage    69.6%   69.62%   +0.02%     
==========================================
  Files          44       44              
  Lines        2441     2443       +2     
==========================================
+ Hits         1699     1701       +2     
  Misses        573      573              
  Partials      169      169

Impacted Files	Coverage Δ
cmd/issue_edit.go	86.92% <100%> (+0.2%)	⬆️

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[rsteube]

Yes that's correct. Tried it out and it works.
(fyi: https://github.com/zsh-users/zsh-completions/blob/master/zsh-completions-howto.org#utility-functions)