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.

* Create `auditory#ToggleMappings()` that turns sounds on if they're off
  and off if they're on
* Create an `:AuditoryToggle` command that runs the toggle function in a
  more user-friendly callable way

With a toggle command, turning Auditory on or off can be as simple as
hitting 1 custom mapping. With our previous `AuditoryOn` and
`AuditoryOff`, you would have to use 2 mappings: 1 for on and another
for off.

Realised that I don't need to explicitly check for 1, I can check
against true or false. Get rid of the explicit check to make this more
concise.

Don't heedlessly turn on sounds when the plugin is loaded. Instead, give
users a choice of whether or not sounds should be loaded when starting
Vim with a configuration variable that can be set in a user vimrc. By
default sounds are now off.

Create a new command to match `:AuditoryOff` that turns on mappings and
sounds. We'll need this because I'm going to make the plugin not enable
sounds by default. That will be controlled by a global variable. You'll
be able to turn the sounds on and off as you wish.

* Move the insert mode autocommand definition into a new function that
  turns on insert mode sounds
* Create a function to turn off insert mode sounds
* Call these from our normal `AssignMappings` and `Unmap` functions so
  we can turn all our sounds on and off in one fell swoop

New command that turns the plugin off by removing its mappings.

Rename the function because we're not only assigning normal mode
mappings here, we're also assigning visual mode mappings. And we might
be assigning mappings for other modes here too in the future, so this
name doesn't make sense.

I had originally called it this to distinguish it from the insert mode
set defined in plugin/auditory.vim with the `CursorMovedI` autocommand,
but I think this should be more clear about what it's doing.

Edit: Also modify the comment above this section for similar reasons.

Assign our normal mode mappings with our new function that uses the
mapping dictionary.