Rename pure.zsh ---> pure.plugin.zsh

This will then follow the conventions for

  * antigen: https://github.com/zsh-users/antigen
  * antigen-hs: https://github.com/Tarrasch/antigen-hs (enforced)

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Feel free to say no to this, but I think we really should strive to standardize all the zsh plugins around github. There were some discussion in zsh-users/antigen#23. The reason I want to change it here is that I just got an email from a user who couldn't get the pure plugin to work with antigen-hs.

Cheers