Make `default` theme the default theme for omf.

* On clean install, configure and download `default` theme.
* Do not allow `default` theme to be removed.
* When the current theme is removed, fallback to `default`.

Add gem package

Gives back a way to persist and update installed packages (dotfiles)

Add rvm package

themes: make 'agnoster copy' back into 'agnoster'

registry: add pbcopy plugin

describe without ruby