Add an 'addons' plugin, which provides heroku style addons.

This is still WIP. I've created the PR so people can comment it.
Add-ons provide access to services (e.g. database) by applications.

It is quite similar to heroku's addons.
Add-ons provide a URL, which is passed to applications through environment variables.
The contents of the url is up to the add-on, but should be described in the add-on's doc.

I use stderr to display error messages, so PR progrium/pluginhook#5 is required to see them.

User interface
-------------------
Four commands are available to manage plugins : 
* `dokku addons` Lists all addons an application has.
* `dokku addons:add` Adds an addon to an application. This cause the addon to allocate resources for this application (eg an account, database, ..)
* `dokku addons:url` Shows the URL a plugin has given. This is only meant for debugging. In production, there's no need to read this url manually, as it is provided in an environment variable.
* `dokku addons:remove` Removes an addon from an application. This causes the addon to destroy all associated resources.

The URL is passed to the application through the `DOKKU_${ADDON}_URL` where `${ADDON}` is the name of the add-on in uppercase.

Add-on development
---------------------------
Add-ons are located in `/var/lib/dokku/addons/`. Every subfolder is an add-on.
When they provision an app, add-ons should generate a unique ID, and a "private" value.
The ID is used to identify the app within the add-on. This would typically be a username or database name.
The "private" value is used to generate the URL. This would be a password for example.

IDs and private values can take any value. They however must not include semicolons, as it is used internally by the plugin. (See the internal section)

Add-ons must provide three executable files : 
* `$ADDON/provision` This script takes one argument, the app name. It should only be used to generate understandable IDs, which contain the app name. Add-ons shouldn't use it the access the app's config files, ...
The script should output the generated id and private value on stdout, separated by a semicolon.
* `$ADDON/unprovision` This script takes one argument, the ID. It should destroy all resources associated with this ID.
* `$ADDON/url` This script takes two arguments, the ID and the private value. It should output the url on stdout.

Add-on's are free to run the service in the way they like. They can run it on the cloud, on the host, on a single docker container or on a container per provisioned app. The plugin doesn't care, as long as the add-on provides a URL which is accessible.
Because URLs might change (docker can assign different IPs/ports after reboot), the `url` script is called each time the app is released. (BTW, this means we should release all apps at startup, rather than deploy them)

Internals
------------
The add-on plugin uses the `$APP/ADDONS` file to save which add-ons are in use.
Each line has the following format : 

    name;id;private

In the `pre-release` hook, the plugin generates environment variables based on the ids and private values, using the `url` script, and saves them to `$APP/ENV`

TODO
---------
- [x] Basic commands (add/remove/url)
- [ ] Help message
- [ ] Unprovision automatically all add-ons in `pre-delete`
- [ ] Manage add-ons enabling/installation (eg creation of service's container)
- [ ] Write add-ons for major services (mysql/mariadb, postgresql, redis, ...)
- [ ] Do some testing, I haven't done much yet
- [ ] Create a wiki page, which explains how to use / create add-ons (based on what I have written above)

Comments/suggestions are welcome.