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

no dependency on ruby & a little quicker. all credit to @derekstavis for the
cool regexs

describe works on installed packages

also modified list_db_packages to return all packages by default, but with a
skip_installed to skip listing installed packages

Add updated tab plugin