Allow execution of post Erlang/Elixir/App hooks

Add `post_erlang_hook`, `post_elixir_hook` and `post_app_hook` options to `elixir_buildpack.config`.

Create a bash script to execute in your app's root dir (e.g. `deploy/post_app.sh`): 
```bash
#!/usr/bin/env bash

export GEM_HOME="$build_path/gems"
export GEM_PATH=$GEM_HOME
mkdir -p $GEM_HOME || exit 1
LANG="en_US.UTF-8" gem install --no-rdoc --no-ri bundler || exit 1
export PATH="$PATH:$GEM_PATH/bin"
cd $build_path
bundle install || exit 1
bundle exec compass compile || exit 1
```

Specify the script you want to be executed in your `elixir_buildpack.config` file:
```
post_app_hook=deploy/post_app.sh
```

Enjoy! :smile: