vol command improvements

[ericmarkmartin]

Added check that volume argument is a number if it is not 'up' or 'down' and echo's new volume level when setting to a specific volume level

Added more helpful text for improper vol option

[tpritc]

That same as #44, could you update the vol show command to just be vol in your help text? Thanks.

[ericmarkmartin]

As a side note, now that spotify vol show no longer shows the volume:

$ spotify vol show
Improper use of `vol` command
The `vol` command should be used as follows:
  vol up                       # Increases the volume by 10%.
  vol down                     # Decreases the volume by 10%.
  vol [amount]                 # Sets the volume to an amount between 0 and 100.
  vol                          # Shows the current Spotify volume.

this is technically an API breaking change. I don't know if you follow semver or anything but when this is merged it would be classified as a major version.

[hnarayanan]

This line appears to be removed by accident.