Add mix-fast plugin (similar to rake-fast)

Signed-off-by: Mikhail S. Pobolovets <styx.mp@gmail.com>

plugins/mix-fast/README.md

+# mix-fast
+
+Fast mix autocompletion plugin.
+
+This script caches the output for later usage and significantly speeds it up.
+It generates a .mix_tasks cache file for current project. Currently if you want
+to update cache you should remove .mix_tasks file
+
+Inspired by and based on rake-fast zsh plugin.
+
+This is entirely based on [this pull request by Ullrich Schäfer](https://github.com/robb/.dotfiles/pull/10/), which is inspired by [this Ruby on Rails trick from 2006](http://weblog.rubyonrails.org/2006/3/9/fast-mix-task-completion-for-zsh/).
+
+
+## Installation
+
+Just add the plugin to your `.zshrc`:
+
+```bash
+plugins=(foo bar mix-fast)
+```
+
+You might consider adding `.mix_tasks` to your [global .gitignore](https://help.github.com/articles/ignoring-files#global-gitignore)
+
+## Usage
+
+`mix`, then press tab
+
+Currently maintained by [styx](https://github.com/styx/)
\ No newline at end of file

plugins/mix-fast/mix-fast.plugin.zsh

+_mix_refresh () {
+  if [ -f .mix_tasks ]; then
+    rm .mix_tasks
+  fi
+  echo "Generating .mix_tasks..." > /dev/stderr
+  _mix_generate
+  cat .mix_tasks
+}
+
+_mix_does_task_list_need_generating () {
+  [ ! -f .mix_tasks ];
+}
+
+_mix_generate () {
+  mix --help | grep -v 'iex -S' | tail -n +2 | cut -d " " -f 2 > .mix_tasks
+}
+
+_mix () {
+  if [ -f mix.exs ]; then
+    if _mix_does_task_list_need_generating; then
+      echo "\nGenerating .mix_tasks..." > /dev/stderr
+      _mix_generate
+    fi
+    compadd `cat .mix_tasks`
+  fi
+}
+
+compdef _mix mix
+alias mix_refresh='_mix_refresh'