A single shortcut to extend the window to the bottom of the screen.

Change the modifiers to match the window movement hotkeys.

Takes the shortcuts of the previous medium increment bindings. Medium
increments now use the Shift modifier, and small increments use Option.

Used the following Spoons as a format reference:

- https://github.com/Hammerspoon/Spoons/blob/master/Source/Tunnelblick.spoon/init.lua
- https://github.com/Hammerspoon/Spoons/blob/master/Source/Token.spoon/init.lua

The Spoons documentation describes how to generate a required
`docs.json` file:
https://github.com/Hammerspoon/hammerspoon/blob/master/SPOONS.md

Thanks to Adam Matan (https://stackoverflow.com/users/51197/adam-matan)
on Stack Overflow for the tip on Make's `CURDIR` variable. Didn't know
that existed.
https://stackoverflow.com/questions/52437728/bash-what-is-the-difference-between-pwd-and-curdir/52447088#52447088

Convert the script to a Hammerspoon Spoon. Allows a user to set a custom
binding to activate the mode.