Its helpful to have an auditive/visual cue when a burn operation
completed.
Instead of adding a setting entry to enable/disable notifications, you
can use the standard way to control notifications from your operating
system. For example, in OS X, you might go to "System Preferences" ->
"Notifications" and disable notifications for "Etcher".
Fixes: https://github.com/resin-io/etcher/issues/280
Signed-off-by: Juan Cruz Viotti <jviottidc@gmail.com>
