Add an `update` routine to the core

[elamperti]

It would be useful to be able to update the dotfiles and its related bundles and processes, so everything is kept updated.

Use cases:

• When new features are added to the core of the dotfiles' setup it may not be called by an existing user unless it runs the setup process again.
• Third-party files are constantly being improved or updated (e.g. fonts, Vim plugins).
• Bundles could have an update() function (implies we know which bundles were installed -and maybe that there should be an uninstall() function too- 😳).

Considerations:

• The update process (and the setup itself, hopefully) shouldn't override existent configurations nor download files if it's not necessary.