Add zsh-completions

The [zsh-completions](https://github.com/zsh-users/zsh-completions) repo
contains completions for various commands but not all of these are
useful so the `update-completion-links.zsh` script inspects the `PATH`
to determine which of the completions should be symlinked to
`~/.local/share/zsh/site-functions` which resides on the `fpath`.

.conduit.yaml

@@ -29,10 +29,13 @@
     - https://github.com/zsh-users/zsh-autosuggestions.git
     - https://github.com/zsh-users/zsh-history-substring-search.git
     - https://github.com/zsh-users/zsh-syntax-highlighting.git
+    - https://github.com/zsh-users/zsh-completions.git
     - https://github.com/junegunn/fzf.git
 - command:
     - fzf/install --bin
 - symlink:
     - {src: fzf/bin/fzf, dst: ~/.local/bin/fzf}
     - {src: fzf/bin/fzf-tmux, dst: ~/.local/bin/fzf-tmux}
+- command:
+    - zsh update-completion-links.zsh
 - message: zsh will be the default prompt after next login

update-completion-links.zsh

@@ -0,0 +1,19 @@
+#!/usr/bin/env zsh
+
+# Check if third party completions are present.
+local zsh_completions=~/.config/zsh/zsh-completions
+[ ! -d $zsh_completions ] && return 0
+
+# Loop over all completions.
+for completion in $zsh_completions/src/_*; do
+  local name=`basename $completion`
+  local symlink=~/.local/share/zsh/site-functions/$name
+  # Remove existing completion file if it exists.
+  [ -f $symlink ] && rm $symlink
+  # Check if the command exists on the PATH.
+  if which ${name:1} &> /dev/null; then
+    # Symlink the completion for the existing command.
+    [ `uname` = Darwin ] && \
+      ln -s $completion $symlink || ln -sr $completion $symlink
+  fi
+done