Symlink pure.plugin.zsh ---> pure.zsh

This will then follow the conventions for

  * antigen: https://github.com/zsh-users/antigen
  * antigen-hs: https://github.com/Tarrasch/antigen-hs (enforced)
Merge pull request #75 from Tarrasch/symlink-plugin-zsh

Symlink pure.plugin.zsh ---> pure.zsh