The shortcuts don't work yet unfortunately. Just committing this as a
progress commit.

Very messy at the moment. Doesn't currently work because it needs to be
executed when the pager view becomes visible. Pager divs are created and
removed each time a message is opened.

Going to put this aside for now and try to focus on the index view. From
there, I should be able to figure out how to load in pager commands when
a message is selected.

Since I'd like to put non-class selectors in this module now.

Instead of using a timeout, which may get executed prematurely, check
for the visibility of Gmail's `loading` screen. When it disappears from
view, we know we can safely initialise all our plugin features.

This allows us to, for example, try to ensure our `<style>` element gets
appended last. Now the sidebar actually gets hidden with our CSS rule
from 496ac03b0d6afb28db09998eb9944db2102ad49e since it can take
precedence over Gmail's asynchronously-loaded styles.

Make the multi-key command generic so that it works with other commands,
not just `\m`.

Use the array equality method from Abhi
(https://stackoverflow.com/users/2968762/abhi) on Stack Overflow:
https://stackoverflow.com/questions/7837456/how-to-compare-arrays-in-javascript/19746771#19746771

Move this to a separate module as we'll probably want to use the same
dictionary of key codes to make other shortcuts.

Also fix some syntax while we're at it.