Add a plugin from command line

[tbruyelle]

Currently to add a plugin the user has to edit his chain/config.yml, and then runs any ignite command to fetch and compile the plugin.

A more convenient way can be achieved by adding a new ignite plugin add <path> command, that would add the plugin in the config file, and then fetch and compile it.

ignite plugin remove <path> can also be considered, to remove the plugin from the config file, and eventually clean the plugin data located in .ignite/plugins/<path>.