Refactor the installer to be more modular instead of being a mess about dotfiles-framework HOT 2 OPEN

This can more or less be considered "done" by whisperity/dotfiles@2c7eb85 at the end of refactoring.

Keeping issue open because the "action items" a package descriptor can take is still to be done.

• Create sensible action item names, arguments, etc. (e.g. shell-tryinorder sounds ugly)
• Configuration variables for a package should go into a new top-level key, as opposed to being a prepare action
  • Let users specify values to such config opts on the command-line
• Document action items in README.md

from dotfiles-framework.

The last remaining bit is tied to the refactoring of what was implemented for whisperity/dotfiles#15, too.

from dotfiles-framework.