Make alert function advisable + fix advice lib

[jaidetree]

Fixes a small inconsistency in the advice defadvice macro + makes the modal function advisable. Includes updates to the tests to catch issues like this in the future.

Fixes #74
Closes #75
Closes #95

Then you can add the following to your config.fnl:

(require-macros :lib.advice.macros)

(defadvice alert-all-screens
           [str style seconds]
           :override alert
           "Displays an alert across all screens"
           (pprint {:str str
                    :style style
                    :seconds seconds})
           (each [key screen (ipairs (hs.screen.allScreens))]
             (hs.alert.show str
                            style
                            screen
                            seconds)))

Then the spacehammer modal will display on all screens. @Grazfather may create a follow up PR to add it to the docs as an official, practical example.

The exact cause was due to use the metatables to make the advisable functions callable while storing references to their advice entry and key.