[RFC] Better config support

Add better support to config support. It's the beginning of the implementation of

https://trello.com/c/WsDMg7RP/161-initial-support-for-avocado-config-files-server-and-client

As a matter of fact, avocado has support for config files, it was only not very visible, and we did not create the config file automatically in the user's config dir. This patch changes that, and adds a new plugin for config values configuration and visualization.

In the process of writing this new plugin, I took on the task of improving the current plugins, and streamline them a bit (split some plugins to different files and visual output tweaks).