Previously executing `:AuditoryOn` and `:AuditoryOff` would always do
the thing they're supposed to do regardless of whether Auditory was
already on or off.

This caused errors because things got messed up when trying to
double assign the mappings and double unmap the mappings.

Now check for the status of the `g:auditory_on` flag before turning
Auditory on or off.

Pressing `G` would bring the cursor to the first line in the buffer
instead of the last line in the buffer because the count in the mapping
was always `v:count1`. This meant that the `G` mapping behaved like `1G`
when no count was specified.

This change fixes the `G` mapping to use `v:count` so that it goes to
the last line in the buffer as expected.

We also add support for arbitrary mappings to choose whether they want
to be `v:count` or `v:count1` depending on what value is specified in
the mapping dict.

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.

When using `:AuditoryToggle` in conjunction with `:AuditoryOn` or
`:AuditoryOff`, things would get messed up. This is because
`:AuditoryToggle` relies on `g:auditory_on` in order to know whether to
turn sounds on or off, but the variable was only getting set in
`auditory#ToggleMappings`, not in the functions that `:AuditoryOn` or
`:AuditoryOff` call.

Move the setting of the variable into `auditory#AssignMappings` and
`auditory#Unmap` so that the right thing will happen regardless of which
on/off/toggle commands you run.

* 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.

* 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

Add support for unmapping a map from any mode where it was defined, not
just normal mode mappings from `auditory#Unmap()`.