Add more options to the exec command

[nickserv]

--all

In order to keep certain functionality if we remove git as a runtime dependency, we should add a homesick exec --all flag to make it easier to run commands on every cloned castle.

As an example, here's what the --all flag for homesick pull looks like currently:

method_option :all,
              type: :boolean,
              default: false,
              required: false,
              desc: 'Update all cloned castles'

--pretend and --quiet

This will be especially useful for #99.

[JCook21]

@nicolasmccurdy I have some code that adds this functionality, just need to add some tests. I'll hopefully get a PR together during this weekend.

[nickserv]

@JCook21 fixed this in #105. His implementation uses an exec_all command instead of an --all flag, which makes more sense when dealing with castle names. However, in my opinion, specifying castle names should eventually be done with a homesick-wide flag, which means that exec_all could change into an --all option for exec instead.