New 'enabled' property on displays to prevent slide rendering

[avanwinkle]

This PR adds a new configuration option enabled to the displays: configs. By default a display is enabled but it can be explicitly disabled via config.

displays:
  display_dmd:
    width: 128
    height: 32
  display_lcd:
    width: 640
    height: 480
    enabled: false

This setting allows a single machine codebase to support multiple display configuration variants without having to duplicate all of the slides or double-up all the names. By passing a mpf -c other_config argument in the command line, an alternate configuration that changes the enabled displays can be used to prevent rendering of unneeded slides.

Use case 1: Extended development slides

In this use case, a secondary display can be created and rendered that shows debugging options, variables and machine state, and other data useful to the developer. This display can be enabled on a development machine and disabled on a production machine, improving the slide render time on the production machine.

Use case 2: Multiple machine variants

In this use case, a game can be developed that offers different machine configurations: one edition with a DMD and another with an LCD, or one edition with an in-playfield screen and another version without.

[avanwinkle]

Build failed because the sound card failed to initialize on the test runners :(

[jabdoa2]

Don't think the test failure is sound related. Those errors always happen. Tests still pass.

[jabdoa2]

LGTM!