Add an 'add-ons' plugin, which provides heroku style add-ons.

Add-ons provide access to services (e.g. database) by applications.
This is still WIP.
Add an ADDONS.md file to describe how to use the plugin.
Fix indentation, refactor app/addon checking and add add-on private directory.
Add a help message to the add-on plugin.
Sort add-ons list and remove trailing whitespaces in pre-release.
Unprovision all add-ons in 'pre-delete'.
Fix a bug where the add-ons plugin would delete all ENV file.
Add enable/disable to the add-on plugin.

Add-ons must be enabled before they are used. This allows add-ons to
install dependencies, allocate necessary resources, ...
export ADDON_ROOT in the pre-release hook
Refactorize a lot of code, and rename ADDON_ROOT to ADDON_DATA.
Replace the semicolon by a colon in the ADDONS file.
Add a few comments in addons-common.
Move and split the ADDONS.md file.
Add the addons directory to gitignore.