Make the mappings dictionary global to allow it to be user-customisable.
This way users can add new commands or sounds to be mapped without
modifying the plugin directly. Seems like it could be useful.

Delete some old code that isn't used any more. This was when I first
made the plugin. These functions were used to start a song when insert
mode was activated and stop the song on insert leave.

Since we're not playing full songs any more, and our insert sounds are
tied to key presses, these functions are no longer needed.

My change from 71ec214cdb8814ecb71c837bf947583fda54d37e caused Galaxy
Far Far Away sounds to be turned on any time you had
`g:auditory_galaxy_far_far_away` turned on, regardless of whether
`g:auditory_on` was also turned on.

Fix this because we never want sounds to play when `g:auditory_on` is
off.

We now rely on `auditory#AssignInsertMappings()` to know whether to use
Galaxy Far Far Away or default insert mode sounds.

Need to toggle `g:auditory_galaxy_far_far_away` in that function because
we're calling the toggle function so we need to set it to the opposite
of what we want.

In the future if we add more insert mode sounds we'll have to change
this structure around but it should work for now.

Adding a configuration variable for these sounds allows them to be
turned on by default without having to
`:AuditoryToggleGalaxyFarFarAway`, so if you prefer these sounds you can
enable them at the outset.