Add simple Hotkey Mode functionality

This adds a simple mode.lua library to Hydra.

Example usage, using the grid.lua library:

```lua
moom = ext.mode.bind({'shift', 'cmd'}, 'M',
   function() hydra.alert('Move Windows', 3) end,
   function() end)

moom:bind({}, 'ESCAPE', function() moom:exit() end)
moom:bind({}, 'w', ext.grid.pushwindow_up)
moom:bind({}, 'z', ext.grid.pushwindow_down)
moom:bind({}, 'a', ext.grid.pushwindow_left)
moom:bind({}, 's', ext.grid.pushwindow_right)
moom:bind({}, 'm', ext.grid.maximize_window)
```

Closes #138