Add styler zsh completions

Can list available commands, specific options for commands (such as
list), and a complete list of installed themes that can be applied.

styler/.config/zsh/completions/_styler

@@ -0,0 +1,50 @@
+#compdef styler
+# the above line should contain all commands to be completed using this file
+# zsh autocompletions for styler script
+
+local curcontext="$curcontext" state line ret=1
+local cmdnames=(theme set download list help version)
+
+_arguments -C \
+  '1: :->cmds' \
+  '2: :->args' \
+  '3: :->subrout' && ret=0
+
+local -a initial_commands=(
+  'theme:Temporarily switch theme to specified argument'
+  'set:Permanently set theme to specified argument'
+  'download:Download a base16 compatible template'
+  'list:Display installed processors, templates, or themes'
+  'help:Display help text and exit'
+  'version:Display styler information and exit'
+)
+
+__themes() {
+  echo "$(styler list themes)"
+}
+
+case $state in
+cmds)
+  _describe -t commands 'styler cmd' initial_commands && ret=0
+  ;;
+
+args)
+  case $words[2] in
+  theme | set)
+    _values 'themes' $(__themes) && ret=0
+    ;;
+
+  list)
+    _values 'listopts' \
+      'processors[Display available application processors]' \
+      'templates[Display available base16 templates]' \
+      'themes[Display available base16 themes]' && ret=0
+    ;;
+
+  *)
+    ((ret)) && _message 'no more arguments'
+    ;;
+
+  esac
+  ;;
+esac