Add a "run-hook" command

[chasinglogic]

You should be able to run hooks directly via a run hook command. This allows you to run setup scripts on systems without necessarily incurring the side effects of the primary dfm command. For instance I use both Dropbox and Git for my dotfiles but need to run my "after_clone" hooks to install spacemacs despite not needing to clone my dotfiles on this new machine.

Syntax should be:

dfm run-hook <hook_name>

For the example above:

dfm run-hook after_clone

It should accept multiple hooks and run all of them.