Fix zsh completion to directly import in fpath

[OmeGak]

My problem is that completion/boom.zsh cannot be imported to zsh's fpath, since it doesn't follow the _* name scheme.

Although I could copy&paste&rename the file to my fpath, I would like to be able to import the file directly from GitHub using zplug. In my PR, I'm renaming the boom zsh completion to _boom so that zsh picks it up. Also, moving it to a completion/zsh directory that can be added to fpath. For backwards-compatibility purpose I keep a symlink with the same name in the original location.

The reason I would like this merged is so that I can install the completion script from zplug like this:

zplug "holman/boom", from:github, \
  use:"completion/zsh